This isn't really anything to do with OP's issue, but I'll try and take a stab at an explanation of your thing.

The first time an instance "knows" about a community hosted elsewhere is the first time a user searches for it through that instance. If they subscribe to it, the community will also now show up in that instance's "All" feed.

So your question naturally follows. If your instance doesn't know about communities elsewhere until the first person tells it, how can you find communities your instance doesn't know about yet?

That's where tools like Lemmyverse come in, this let's you search for communities hosted anywhere.

Hope that makes sense, writing this from my phone so excuse any typos etc.