If you search for a community for the first time (no user ever subscribe to that community in your instance), 20 posts are fetched initially.
After you subscribed, the community will send updates to your instance. So new posts, comments, etc will be federated going forward.
If you want to pull older posts or comments to your instance, you can paste their url into the search field and wait a few seconds to pull them. If you’re pulling a comment, sibling comments are not fetched, only parent comments got included automatically.
According to their documentation:
https://join-lemmy.org/docs/administration/federation_getting_started.html