I just crossposted something using Thunder and another user let me know that my crosspost did not show that the other post was the original.
See here: https://lemmy.ca/comment/11901004
Not sure how that can happen but seems like a bug in Thunder then?
There is no special backend data, and lemmy makes no distinction between which post is “original” aside from the difference in the timestamp.
Lemmy simply hooks up all posts into cross-posts that link the same URL.