Communities can be on relays.
You have a master community npub as the owner
Then the master owner can publish a set of moderator/manager keys
Community data can be on any relay
Update:
nostr:npub12262qa4uhw7u8gdwlgmntqtv7aye8vdcmvszkqwgs0zchel6mz7s6cgrkj changed my mind on sth.
Communities can be on relays.
You have a master community npub as the owner
Then the master owner can publish a set of moderator/manager keys
Community data can be on any relay