@datatitian @activitypub To post in a Lemmy community, you mainly need to include its id in the to/cc field. This works from most platforms including Mastodon. Replies to posts appear on Lemmy automatically, but you should again put the community it in to/cc. Other than that you can also like/dislike, follow communities and more.
This FEP has a general overview: https://codeberg.org/fediverse/fep/src/branch/main/feps/fep-1b12.md
I am planning to rewrite the federation docs soon, do you know any good example that I can look at?