#ActivityPub is super cool once you see it in action.
For instance, you can visit the new StarTrek lemmy server here: https://startrek.website/c/startrek. Looks like a reddit sub with posts, threaded comments, upvotes, &c.
OR you can follow the same server on Mastodon @startrek.
Every thread and comment shows up as a boosted post.
Cool, right? Now, say you find a comment that you want to reply to. Post through your favorite Mastodon app, and that feeds right back to the thread on the #Lemmy server! 🤯
Yeah they DEFINITELY aren’t the same communities. For lemmy and kbin instances (the two reddit-like softwares on the fediverse) communities are like subreddits but they are subreddits specific to that particular website/instance (startrek.website, lemmy.wordl). Many different lemmy instances/websites (and kbin instances/websites for that matter too) can have their own /c/politics, they are entirely separate entities that just happen to share a name.
A bit confusing, not gonna lie lol, but I think it ultimately makes sense as a way to structure things.
Also kbin calls subreddits “magazines” and an example of a subreddit on a kbin instance is https://kbin.social/m/gaming