To add and reiterate what others have said:

• It’s similar to following sports and knowing all the game results, who’s injured/out for the season, etc.
• People like to feel part of larger society/culture, and to feel knowledgeable, to feel “in on things”
• It’s a way to fill time/fill the void, a distraction from “real life” which can leave you feeling powerless/drained
• It can be a good conversation topic with friends when there’s not much to talk about (or when other topics can be contentious)

Some ways to try to revert it (some have already been mentioned)

👉 Go to this page and check if you’re able to opt out of the “Redesigned Watch Page” (source)

👉 Install a userscript addon like Violentmonkey, and then use a script like Classic Youtube Layout (which is based on this script which was outdated in regards to the related videos sidebar)

• If you get an error about the @match line of the script then click on allow edits and change it from *.youtube.com/* to *://*.youtube.com/*, or install the script as it is and then put *://*.youtube.com/* in the “@match rules” line in the settings tab of the script
• There’s also this script that you would need to add its code manually to the userscript addon by clicking on the new script icon (see the author’s post for more details)

👉 Use uBlock Origin filters like these or these

• Note that youtube.com##+js(set, yt.config_.EXPERIMENT_FLAGS.kevlar_watch_grid, false) should be enough to change the UI back for the most part

👉 Use this userscript and Stylus style

👉 The addon Enhancer for YouTube (Firefox / Chrome) also reverts the UI, at least partially

👉 (might only work some times) Use the addon CustomTube - Firefox / Chrome (the default UI is from 2017 but you can choose 2021 in the settings. It’s not exactly like the one from 2023 but it’s close enough)

• note that it removes some buttons like Join by default so go over the settings if relevant

If you also want to change the number of videos per row in the subscription page and channels’ videos page, then you can add this uBlock Origin code to the “My filters” tab (at the end click “Apply changes”, or Ctrl+S) (this is for 6 items per row but you can change the number in the first 2 line as needed) (source 1 / source 2)

youtube.com##ytd-rich-grid-renderer:style(--ytd-rich-grid-items-per-row: 6 !important;)
youtube.com##ytd-rich-grid-renderer:style(--ytd-rich-grid-posts-per-row: 6 !important;)
youtube.com##ytd-two-column-browse-results-renderer.grid-6-columns:style(width: 100% !important;)
youtube.com##ytd-rich-grid-row, #contents.ytd-rich-grid-row:style(display:contents !important;)
youtube.com##ytd-two-column-browse-results-renderer.grid:not(.grid-disabled):style(max-width: 100% !important;)

Sorry, it was unclear in my comment. By “it’s actually part of the Mozilla Corporation” I was referring to Firefox, not Mozilla Foundation

for anybody that wants to disable it, go to the settings and search for “Allow websites to perform privacy-preserving ad measurement”

(or through the dom.private-attribution.submission.enabled flag in about:config)

https://support.mozilla.org/en-US/kb/privacy-preserving-attribution

Just adding that as I understand this, donations to the Mozilla Foundation cannot go towards Firefox, because it’s [edit: Firefox is] actually part of the Mozilla Corporation. To help with funding Firefox people can consider purchasing the Corporation’s other products (VPN/Relay/Monitor), or purchasing merch.

See more here on the AMA on Reddit, and this thread

• Lights: Switching to light sources/bulbs with lower brightness/warmer light temperature (lower Kelvin/K-value) 3 hours before bed. (Light affects our sleep-wake cycle, aka the circadian rhythm. Our brain processes bright light as “the sun is still up so it’s not time to sleep yet”)
• Screens: Gradually dimming electronic screens until bedtime (Computer: using F.lux and lowering the brightness gradually in the graphics card’s control panel. Phone: with a built-in Night/Dark Mode option that you can schedule or by using an app like Twilight). // Don’t take your devices to the bedroom, or keep them far away from the bed and set them on silent/shut them off.
• Try not to sleep for more than 8 hours so you’ll need the sleep the next night.
• Sleeping in a closed and completely dark room (including covering LED lights on electronics, or using a face mask) with something that makes a white noise (fan/air purifier/etc.)
• Food: Having a light meal 3-3.5 hours before bed and light snacks 1.5-2 hours before bed, to not put the body into overdrive digesting a heavy meal or cause heartburn/indigestion, and also to not go to sleep hungry and get distracted by that.
• If you sleep on your stomach with your head to the side, you can use a thin and narrow side pillow (but firm) along your torso to support your shoulder on the side that your head is pointing to. (you can also use a thin pillow for your head to not strain your neck/spine - there are ones made for kids if needed)

• Lights: Switching to light sources/bulbs with lower brightness/warmer light temperature (lower Kelvin/K-value) 3 hours before bed. (Light affects our sleep-wake cycle, aka the circadian rhythm. Our brain processes bright light as “the sun is still up so it’s not time to sleep yet”)
• Screens: Gradually dimming electronic screens until bedtime (Computer: using F.lux and lowering the brightness gradually in the graphics card’s control panel. Phone: with a built-in Night/Dark Mode option that you can schedule or by using an app like Twilight). // Don’t take your devices to the bedroom, or keep them far away from the bed and set them on silent/shut them off.
• Sleeping in a closed and completely dark room (including covering LED lights on electronics, or using a face mask) with something that makes a white noise.
• Food: Having a light meal 3-3.5 hours before bed and light snacks 1.5-2 hours before bed, to not put the body into overdrive digesting a heavy meal or cause heartburn/indigestion, and also to not go to sleep hungry and get distracted by that.
• If you sleep on your stomach with your head to the side, you can use a thin and narrow side pillow (but firm) along your torso to support your shoulder on the side that your head is pointing to. (you can also use a thin pillow for your head to not strain your neck/spine - there are ones made for kids if needed)

On the humorous side:

• My Brother, My Brother And Me - 3 brothers giving not-so-serious advice to listeners, reacting to questions from Yahoo Answers (when that was a thing), and later adding other segments, and overall being funny/silly (note: sometimes they make references that go over my head, but they’re still enjoyable. These references might be best enjoyed by midwestern Americans who grew up in the late 80’s/early 90’s)
• My Dad Wrote A Porno - 3 people reacting to an “erotica” book series that one of the guys’ dad published (I would say the books are maybe only 5%-20% explicit, depending on the chapter) (some of the characters’ voices can be annoying/grating, and the narrator tends to repeat sentences after they react to them which can be annoying as well)
• No Such Thing As A Fish - the behind-the-scenes staff of the show QI bring up interesting facts and tidbits from history/nature/etc. (each episode is split into 4 parts where each member brings up a fact and the others react to it and bring up related facts)
• If I Were You - Jake & Amir from CollegeHumor giving advice to listeners (mostly in a sarcastic/tongue-in-cheek way but sometimes genuinely), mostly about relationships/dating
• SmartLess - Jason Bateman and Will Arnett (Arrested Development) and Sean Hayes (Will & Grace) interview a famous person each episode where only one of the hosts known who it is beforehand (it gets better after the first few episodes, though some conversations are less funny/entertaining than others)
• Conan O’Brien Needs A Friend - Conan interviews a famous person each episode (I like the interview part better than the introduction, and similarly to Smartless some conversations are less funny than others)
• If you watched The Office (US) - Office Ladies - Jenna and Angela from the show reacting to each episode of the show and bringing up behind-the-scenes stuff (some of episodes include interviews with other cast members/staff) (might not be as humorous as the others but still fun to listen to)

On the humorous side:

• My Brother, My Brother And Me - 3 brothers giving not-so-serious advice to listeners, reacting to questions from Yahoo Answers (when that was a thing), and overall being funny/silly
• My Dad Wrote A Porno - 3 people reacting to an “erotica” book series that one of the guys’ dad published (I would say the books are maybe only 5%-20% explicit, depending on the chapter) (some of the characters’ voices can be annoying/grating, and the narrator tends to repeat sentences after they react to them which can be annoying as well)
• No Such Thing As A Fish - the behind-the-scenes staff of the show QI bring up interesting facts and tidbits from history/nature/etc. (each episode is split into 4 parts where each member brings up a fact and the others react to it and bring up related facts)
• If I Were You - Jake & Amir from CollegeHumor giving advice to listeners (mostly in a sarcastic/tongue-in-cheek way but sometimes genuinely), mostly about relationships/dating
• SmartLess - Jason Bateman and Will Arnett (Arrested Development) and Sean Hayes (Will & Grace) interview a famous person each episode where only one of the hosts known who it is beforehand (it gets better after the first few episodes, though some conversations are less funny/entertaining than others)
• Office Ladies - Jenna and Angela from The Office (US) reacting to each episode of the show and bringing up behind-the-scenes stuff (some of episodes include interviews with other cast members/staff)

On the comedy side like MBMBAM:

• My Dad Wrote A Porno - 3 people reacting to an “erotica” book series that one of the guys’ dad published (I would say the books are maybe only 5%-20% explicit, depending on the chapter) (some of the characters’ voices can be annoying/grating, and the narrator tends to repeat sentences after they react to them which can be annoying as well)
• No Such Thing As A Fish - the behind-the-scenes staff of the show QI bring up interesting facts and tidbits from history/nature/etc. (each episode is split into 4 parts where each member brings up a fact and the others react to it and bring up related facts)
• If I Were You - Jake & Amir from CollegeHumor giving advice to listeners (mostly in a sarcastic/tongue-in-cheek way but sometimes genuinely), mostly about relationships/dating
• SmartLess - Jason Bateman and Will Arnett (Arrested Development) and Sean Hayes (Will & Grace) interview a famous person each episode where only one of the hosts known who it is beforehand (it gets better after the first few episodes, though some conversations are less funny/entertaining than others)
• Office Ladies - Jenna and Angela from The Office (US) reacting to each episode of the show and bringing up behind-the-scenes stuff (some of episodes include interviews with other cast members/staff)
• Conan O’Brien Needs A Friend - Conan interviews a famous person each episode (I like the interview part better than the introduction, and similarly to Smartless some conversations are less funny than others)

Not sure if they all fit entirely but:

• The Story Of Stuff (Annie Leonard)
• How The World Works (Noam Chomsky)
• Predictably Irrational: The Hidden Forces That Shape Our Decisions (Dan Ariely)
• The Hidden Brain (Shankar Vedantam) / Idiot Brain (Dean Burnett)
• The Myth Of Choice (Kent Greenfield) / The Paradox Of Choice (Barry Schwartz)
• The Free Will Delusion: How We Settled For The Illusion Of Morality (James B. Miles)
• Getting Free: Creating An Association Of Democratic Autonomous Neighborhoods (James Herod)
• The Best That Money Can’t Buy (Jacque Fresco)
• No Contest: The Case Against Competition (Alfie Kohn)

Especially for sleeping issues, but also for eye strain, I recommend using a program like flux (for a computer) that allows you to set the color temperature for the day and the gradually lower it further at evening and night. This along with dimming or using different lower temperature lightbulbs in the evening/night (not being exposed to bright/florescent lights) really helped me with falling asleep.

edit: also, lowering the brightness on the monitor itself (mine is set to 27) and in the graphics card control panel (39 for me), and then manually lowering it further in the evening/night in the control panel (25>15>9>0 is how I do it)