According to their documentation:

• 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.

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

🧟John is a 🍎lemm.app user, he subscribes to 🐢turtle community on 🍌lemm.ban

he is the first ever to do this on 🍎lemm.app

so 🍎lemm.app creates a copy with the last 20 posts and now will always keep in sync with future posts

👩‍🚀Jill is also a user on🍎 lemm.app, she subscribes to the same 🐢turtle community but a year later.

she will be able to see all the posts of that year all the way up to those 20 posts.

deleted by creator