Sure, I also have been trying to learn about how Lenmy works. I haven’t yet found a comprehensive overview that details everything though.

From https://join-lemmy.org/docs/administration/federation_getting_started.html

If you search for a community first time, 20 posts are fetched initially. Only if a least one user on your instance subscribes to the remote community, will the community send updates to your instance. Updates include:

New posts, comments Votes Post, comment edits and deletions Mod actions

From: https://join-lemmy.org/docs/users/01-getting-started.html

These previous ways will only show communities that are already known to the instance. Especially if you joined a small or inactive Lemmy instance, there will be few communities to discover.

This issue/post on github has some info: https://github.com/LemmyNet/lemmy/issues/3062

I would also checkout some discussions on [email protected] [email protected] https://selfhosted.forum