You can "follow" communities on a Lemmy instance just like a regular user.
Lets say we're talking [email protected] (and the federation isn't bork).
Search for @[email protected] in Mastodon and (if the federation isn't bork) it should find the community as a user and you can follow.
Followed communities in Mastadon:
- all posts and comments show in an amorphous stream as individual toots -- depending on what interface you use it could be just random toots or they're displayed threaded
- they all show as boosts from the community "user"... which if you think about it, that's all a community does - it reboots all posts and comments-on-posts to all subscribers of the community.