Due to the way federation works, you won't see content generated from before the first time someone on your instance subscribed to the remote community. Once someone on your instance has subscribed to it, then it will federate all future content to your instance
You can pull specific content over though. If you see something on lemmy.ml that you want to reply to, that isn't on your instance, if you copy and paste the URL of the comment in to the search box on your instance, it will pull that post across for you to interact with.
I thought that too but that's not true. Once I subscribed to a community on my instance I could see posts from weeks and months ago... even going back years!
That's because someone else on your instance had subscribed to before you did. Content starts federating from the moment the first person subscribes to a remote instance
Ah, yeah, I know what you mean now. I do remember the devs saying that the first time a remote group profile is pulled, that the API returns the last 20 posts or something. It's a different mechanism to subscription, but it makes sense, because subscription requires the group profile to be pulled first