In your comments you actually used singular they several times probably without realizing it, because singular they has existed in English since the 1300s. Singular you wasn’t even really a thing until the 1600s, so that’s actually the less established pronoun in English.
The other thing is no-one “assigns themselves” or “makes up” their gender. Sure the labels given to things are made up, but so is every other word. Non-binary identities also are as old as binary ones. Some indigenous north American tribes have “two-spirit”. Rome had the Galli, as sort of trans-adjacent priests of Cybele. The jewish talmund lists 7 distinct genders. Babylon had multiple non-binary identies, often as priests of Inanna. Etc.
Irregardless though, the existence of multiple labels to describe similar but not identical concepts isn’t really “redundant”, in the same way that you wouldn’t claim the word “angry” shouldn’t exist because “mad” already does. To you it might not matter what someone identifies as, but its also not your job to decide whether or not that identity “makes sense”, because, by definition, if someone uses a certain label then that label has a reason to exist. No one’s asking you to memorize every single microlabel. That’s why non-binary is so widely used, its easier to have an umbrella term that’s more widely understood.
Gender isn’t something rigid, just like any other aspect of your own identity, and it would exist whether or not you had words to describe it. So arguing that having those words to describe it is more confusing doesn’t really make sense. In fact, before “they” became as accepted as it is now as a genderless 3rd pronoun, people did try to make a distinct third option to prevent confusion. Stuff like “e/em” for example. But none of them really picked up any widespread traction, because it turns out that most people already used they as singluar sometimes and it was a lot easier to just expand that usage naturally than it was to get every English speaker on-board with new pronouns.
And no one is really going to get mad at people for slipping up, or using they to refer to a group, or anything like that. Trust me, most binary trans people get misgendered often enough as is, and its much worse if you’re non-binary. Being corrected about someone’s pronouns isn’t people “getting mad over people not understanding”, 99% of the time its literally just a correction, same as if you forgot someone’s name or mispronounced it. Just like that, unless you’re intentionally doing it to be an asshole, the vast majority of trans and non-binary people aren’t going to make a big deal about it. Especially because we know that there’s the perception that trans/nb people “overreact to mistakes” so a lot of the time there’s a pressure to try and avoid doing anything that might accidentally reinforce that perception. Which really means if anything it’s a lot more common for a trans person to just not say anything when they’re misgendered than it is to say something.
Edit to say:
Also honestly, you probably won’t be able to understand what it actually is when someone is nonbinary. Everyone, regardless of if they’re cis or not has a unique relationship to their own gender. Two cis men will have very different explanations of what being a man means to them. The same is true for any gender identity. Without actually having experienced dysphoria, or gender euphoria, any of the numerous experiences attached to being trans or nonbinary, it can be hard to “get it”. That doesn’t make it less real though, its just not something you’d personally feel, which is fine. No one expects you to be able to. A man also wouldn’t necessarily “get” what being a woman is and vice versa, but that doesn’t make “man” and “woman” invalid identities. I’m a trans woman and I honestly don’t really “get” what being a man is like, because my closest reference is the dysphoria that came from perceiving myself that way, which isn’t what “being a man” is, it was just part of my specific expierience of being trans. That doesn’t stop me from understanding that men exist, or make me consider “man” as a less valid identity, because whether or not I can relate to specific aspects of anyone’s identity as a man is irrelevant to whether or not they feel that it is an accurate or useful label to describe themselves. The same things are true for nonbinary identities from a cis perspective.
It would basically make the same amount of sense for me to say “I don’t understand what being a man is, so there isn’t a reason for it to exist, it’s just confusing. And idk why men get mad at me for using ‘she’ to refer to them. All of my friends are girls so its too hard to use something else besides she/her when I interact with someone in that group. I have nothing against men, but they shouldn’t bring that identity outside of their own circles because its too annoying to understand.” as it does for a binary person to say that nonbinary identities are confusing or shouldn’t exist because they don’t personally feel the need for it.
In your comments you actually used singular they several times probably without realizing it, because singular they has existed in English since the 1300s. Singular you wasn’t even really a thing until the 1600s, so that’s actually the less established pronoun in English.
The other thing is no-one “assigns themselves” or “makes up” their gender. Sure the labels given to things are made up, but so is every other word. Non-binary identities also are as old as binary ones. Some indigenous north American tribes have “two-spirit”. Rome had the Galli, as sort of trans-adjacent priests of Cybele. The jewish talmund lists 7 distinct genders. Babylon had multiple non-binary identies, often as priests of Inanna. Etc.
Irregardless though, the existence of multiple labels to describe similar but not identical concepts isn’t really “redundant”, in the same way that you wouldn’t claim the word “angry” shouldn’t exist because “mad” already does. To you it might not matter what someone identifies as, but its also not your job to decide whether or not that identity “makes sense”, because, by definition, if someone uses a certain label then that label has a reason to exist. No one’s asking you to memorize every single microlabel. That’s why non-binary is so widely used, its easier to have an umbrella term that’s more widely understood.
Gender isn’t something rigid, just like any other aspect of your own identity, and it would exist whether or not you had words to describe it. So arguing that having those words to describe it is more confusing doesn’t really make sense. In fact, before “they” became as accepted as it is now as a genderless 3rd pronoun, people did try to make a distinct third option to prevent confusion. Stuff like “e/em” for example. But none of them really picked up any widespread traction, because it turns out that most people already used they as singluar sometimes and it was a lot easier to just expand that usage naturally than it was to get every English speaker on-board with new pronouns.
And no one is really going to get mad at people for slipping up, or using they to refer to a group, or anything like that. Trust me, most binary trans people get misgendered often enough as is, and its much worse if you’re non-binary. Being corrected about someone’s pronouns isn’t people “getting mad over people not understanding”, 99% of the time its literally just a correction, same as if you forgot someone’s name or mispronounced it. Just like that, unless you’re intentionally doing it to be an asshole, the vast majority of trans and non-binary people aren’t going to make a big deal about it. Especially because we know that there’s the perception that trans/nb people “overreact to mistakes” so a lot of the time there’s a pressure to try and avoid doing anything that might accidentally reinforce that perception. Which really means if anything it’s a lot more common for a trans person to just not say anything when they’re misgendered than it is to say something.
Edit to say: Also honestly, you probably won’t be able to understand what it actually is when someone is nonbinary. Everyone, regardless of if they’re cis or not has a unique relationship to their own gender. Two cis men will have very different explanations of what being a man means to them. The same is true for any gender identity. Without actually having experienced dysphoria, or gender euphoria, any of the numerous experiences attached to being trans or nonbinary, it can be hard to “get it”. That doesn’t make it less real though, its just not something you’d personally feel, which is fine. No one expects you to be able to. A man also wouldn’t necessarily “get” what being a woman is and vice versa, but that doesn’t make “man” and “woman” invalid identities. I’m a trans woman and I honestly don’t really “get” what being a man is like, because my closest reference is the dysphoria that came from perceiving myself that way, which isn’t what “being a man” is, it was just part of my specific expierience of being trans. That doesn’t stop me from understanding that men exist, or make me consider “man” as a less valid identity, because whether or not I can relate to specific aspects of anyone’s identity as a man is irrelevant to whether or not they feel that it is an accurate or useful label to describe themselves. The same things are true for nonbinary identities from a cis perspective.
It would basically make the same amount of sense for me to say “I don’t understand what being a man is, so there isn’t a reason for it to exist, it’s just confusing. And idk why men get mad at me for using ‘she’ to refer to them. All of my friends are girls so its too hard to use something else besides she/her when I interact with someone in that group. I have nothing against men, but they shouldn’t bring that identity outside of their own circles because its too annoying to understand.” as it does for a binary person to say that nonbinary identities are confusing or shouldn’t exist because they don’t personally feel the need for it.