You can follow communities from the Mastodon app, but the experience will be "microblog-y" in nature. Communities are represented as users on Mastodon. So, search for @[email protected] on Mastodon. You will see all the posts in the community as "toots," and all comments as replies.
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.
That was easier than I expected. On the Mastodon instance that I use, no previous posts show up (I tried a few communities on other servers). I believe that’s because no one has previously subscribed from that server, so they weren’t federated yet. I’ll watch them over the next few hours.