I can only talk about the old /bun/ part of the story, having been on /bun/ since shortly after its creation. I have no idea where our teenfriends came from, other than 'some altchans I'm not familiar with and probably Discord'.
/jp/ was created in 2008 (slightly before I joined; I joined late 2018) as a split off /a/ because Moot was sick of /a/ being full of Japan-related stuff that wasn't anime or manga. He instituted a strict rule on /a/ that discussion can ONLY be about anime and manga, and forced all miscellaneous Japanese stuff to the newly created /jp/. Early /jp/ was very much in flux, going through several phases of established board culture. This also meant that as board culture evolved, old board culture was being replaced (forcibly, by influx of new posters). Old /bun/ was created by and posted on by (former) /jp/ posters from those earliest board cultures. (The way I see it, /jp/ truly died in 2014 when mods and janitors started enforcing on-topicness, strictly limiting what could be posted on /jp/ and encouraging/enforcing general threads. But that's unrelated to /bun/.)
Old /bun/ had some structural changes (early disagreements on what can be on /general/ and what must be on /photos/, /photos/ growing in scope, the great /general/ wipe turning /photos/ into practically the main board with /general/ barely getting any activity, /projects/ being launched, abandoned and filled with spambots, and eventually breaking), but in terms of board culture and demographic, /bun/ didn't actually change that much from the beginning. It clearly got less active over time so people must have been leaving, and there were some identifiable tourists, but overall it remained remarkably stable despite everything around it changing. What did get lost over time was the spontaneity in thread creation, eventually nearly all activity was concentrated in a handful of fairly broad threads (small thoughts, random quotes, last picture you saved, last thing you fapped to, recommended reading, etc) where discussion about all kinds of things could still pop up. Activity was low but consistent, and moderation was slow but happened eventually. There were occasional spambots posting porn ad threads, sometimes with CP, and while they usually got removed within a day sometimes they remained up for several days.
Somewhere between August 29th, 11:13 UTC and August 30th, 03:14 UTC, the entire site suddenly went down. I went on IRC to tell Aya, the admin, and was told "Well, it looks like Dreamhost got tired of the child porn spam and pulled a plug on the account". The servers were wiped (and all backups were on the servers so those were gone too), and an appeal was rejected because they had a 'zero tolerancy policy' on this. Months passed with nothing to take its place, until on December 25th the admin of new /bun/ decided to revive /bun/. New /bun/ was inactive at first, but it seems the admin decided to invite his friends and that leads us to new /bun/'s current population.
>I'm pretty sure 100% of people who have posted here posted on bun.
I'm entirely sure this is not the case; the posting style of most posters is very un-/bun/-like. A lot of people also seem to be under the impression that /bun/ is to some degree about Touhou, which was never true for old /bun/, it just happened to have a Touhou theme and everybody was familiar with Touhou (because we all came from old /jp/) but the board subject was essentially 'Japan/General', all the stuff old /jp/ was about. I think a few posters may have posted occasionally on old /bun/ in its later years, but I get the impression I may be the only real '/bun/ refugee' here. I hope I'm wrong, or at least that more old /bun/ posters will find this place.
And for the record, I have no clue what these zoomers are talking about. There's clearly some pre-existing culture here that's entirely unrelated to old /bun/. (I do understand >>318 and >>320, but because of post-/bun/ 4chan (and I continue to insist we don't need textual upvotes and downvotes on imageboards, call me a boomer if you must), and I think I recognize >>319 as a reference to an altchan that's likewise unrelated to /bun/.)