The problem with Fedi apps is that they're mostly using a push model not a pull model. If nobody on your instance subscribes to a community, you're never going to see the full contents of the posts and comments there. This by-design but confusing behavior is something which I hope the Lemmy project will work on improving, so you can browse to a niche remote community or post on your instance and you'll see all the content.
The comments are not there, and don't seem to be on any communities I subscribe to from other servers. Am I misunderstanding how it's supposed to work and only posts are federated?
I found this one, but wanted to connect the new [email protected] community and am unable to discover. Might be a federation issue?
Edit: Was able to join. As suspected, not fully federated out yet. To solve: users not on lemmy.world need to use their instance's website (not via an app) and search directly for [email protected] and click into the community & subscribe. Once done, it will begin to propogate to your instance as well, helping spread it to others that may be interested.