This isn’t quite how federation works. And we are only seeing this problem so much because a lot of communities are being created right now with the influx of users. Communities that are established and popular would already be federated to most multi-user servers.

Each server, when you search, can only bring you results from communities it knows about. What each server knows, is limited to the bare minimum. Otherwise, every server would have to download and contain the entire fediverse. A HUGE amount of data, and part of the idea of the fediverse is to only make each node as big as it needs to be.

As such, the way it works is that each server only knows about stuff that at least ONE user has SPECIFICALLY told it about. This way each server only needs to contain stuff that the users on it actually want.

For a community to federate over to another server, one user on that server must search for it specifically using either the url of the community, or its name in the !name@instan.ce format. Once the first user does this, the server will form that connection and bring that community over. This happens community by community.

This way, each server only needs to contain the parts of other servers, that are actually needed.