The federation with Mastodon is mostly one-way: We can’t see or comment on Mastodon posts, but Mastodon users can see and comment on Lemmy posts.

Mastodon’s like Twitter… its posts wouldn’t fit in the Lemmy UI well. Though I hear kbin works well with both Mastodon-style and Lemmy-style posts.

Mastodon allows you to follow users, which Lemmy doesn't. So you can follow users on here but there is, currently, no facility to follow users over there. I imagine this will change eventually.

Thank you for asking this question. I was trying to find some mastadon based communities that do exist using the search on my instance and not seeing them come up. Now I see why. Maybe it's time to live more on kbin.

Activity Pub has a few parts, Lemmy implements the Threaded message part.

Mastodon implements a short messaging (posts) part. Meta's Threads will implement this.

KBin implemented both parts, within KBin you'll see microblog as an option for magazines (communities/subredits). This shows either 'posts' made to the magazine or posts with hastags associated with the magazine.

The posts and threaded message parts have overlap in how they work so mastodon users can see certain threaded messages and comment on them.

I don't know probably some backend things