I have been the first user on my instance to subscribe to a few “remote” communities. It seems like basically nothing those remote communities from before I subscribed is can be viewed on my local instance. However, after subscribing to a community, anything new that happens there including posts and comments from users on third instances will appear.

In cases where someone else from my instance subscribed to a remote community previously, the full content from those communities from the time they subscribed is also available to me.

I may be misunderstanding your question because I never used Mastodon.