All fediverse software (lemmy, kin, mastodon, etc) implements their own version of ActivityPub in slightly different ways. There are cases where the overlap is enough to cross over (lemmy communities and kbin magazines), and some where it doesn’t work/is wonky (lemmy users can’t follow mastodon users).

There’s a more in depth write up here if you’re curious: https://overengineer.dev/blog/2019/01/13/activitypub-final-thoughts-one-year-later.html

Yes, that is one of the features of being federated. Kbin and Lemmy and Mastadon (and others) can all federate with each other, so posts and comments are all shared. It doesn’t really matter if you’re on Kbin or Lemmy, you can see the same communities/magazines and comments for the most part, and interact regardless of which one you are currently logged in to.

Lemmy, kbin, and Mastodon all speak the same underlying protocol – ActivityPub. I’ve found that the best way to think of it is to compare it to email. If I send you an email from my gmail account to your outlook account, it just works (well, mostly, email is a bit of a mess lol) even though the two email clients look vastly different from each other. ActivityPub (and federated protocols in general) are like email, but for twitter/reddit.

There are some different message types (it wouldn’t make sense to present twitter-like content in the context of a threaded forum like Reddit), and not all instance types support all the different message types. I’m using kbin (via kbin.social) and I can see Mastodon content, but I’m not sure if Lemmy has that ability.

Not sure if that was helpful, and I hope that others come and fact check me, but that’s my understanding of it.

Then wait until you find that you can follow Lemmy/kbin communities from Mastodon and comment on Lemmy/kbin posts from your Mastodon account 🤭