Trying to actually browse tumblr: what is the equivalent of commenting on a post? I see no one uses the actual comment button?

Yeah, before comments were added, you just reblogged the post, and added your content in the bottom, so the posts became increasingly nested as they moved through chains of followers.

Unlike Reddit/Lemmy, this system made it very hard to follow the entire conversation, as you could only see whichever specific "branch" reached you via the people you follow, and you had to go digging in the notes of the original post to try to find other response threads. (Which, in the case of truly popular posts with hundreds of reblogs buried among thousands of likes, good luck.)

Oh and you could also edit the original post (and any edits the OP made didn't show up in existing responses as they essentially snapshotted it in the moment of reblog), meaning it was very hard to find the canonical / original versions of famous threads. Especially as people often deleted their accounts, so the only place a thread might be preserved is some never seen reblog of an obscure, abandoned account :D

They improved the UX a bit in recent years (comments, separating reblogs from likes), but I think due to its innate structure Tumblr is still pretty messy for trying to have conversations with more than a single person. It's still better than Twitter lol. And of course Reddit/Lemmy is miles above either.

Yeah, pretty much. Comments can't be replied to: they're more like things you wanna say just to get it put there. Reblogs can be replied to, and promote interaction. They're also why posts from 10+ years ago are still being paraded around

Yep most people use reblogs, it's easier to form and follow a chain of conversation there. A lot of people will also use the tag field in the reblogs instead of using the actual text field.