Not sure about the top and bottom ones, but I’m pretty sure those @a.gup.pe ones are Mastodon “groups”. They’re a reflector that you tag into a post to have it shared to its followers across all instances, rather than just your local instance (as with hashtags).

They’re a workaround for one of the limitations in Mastodon, but they work. I expect they’d appear empty if viewed in Lemmy.

More info here: https://fedi.tips/how-to-use-groups-on-the-fediverse/