An instance only "knows" about a community on another instance once both (a) it has federated with the other instance and (b) someone has explicitly triggered a search for "!communityname@remoteinstancename" on an instance. Like, that community doesn't get added to the list of known communities on federated instances just because someone has created it.

For example, take bbs.9tail.net, a small lemmy instance.

You can see that that it's federated with lemmy.world on its instances page:

https://bbs.9tail.net/instances

And it's only blocked a single instance, lemmygrad.ml.

But (as of this writing, and that could change if someone goes and starts triggering searches for stuff on there), it only has two pages of communities, with 63 (in a quick count) known. There are far more than 63 communities on lemmy.world alone, not to mention on all the other instances that bbs.9tail.net is federated with.

Lemmyverse.net, on the other hand, crawls all the instances it can find and builds a full index. Currently has over 27,000 communities. Once you get a "!community@instance" name from there, you can trigger a search for it on your home instance, and your home instance will learn about it. But until you or someone else does that, your home instance won't know about that community.