It doesn’t get served to all, but it has to be served to a relay which in turn is reachable by all.

While there is a firehose service which publishes all events as they happen, only services built in watching those needs to receive all (moderation services, etc). Everybody else gets a view already filtered and composed by services earlier in line.

A limited scope appview and relay is possible too, you can choose to only serve one community, and then fetch external content on-demand (but this will have the same impacts on latency as Mastodon and lemmy has when opening previously unseen threads)