Nostr: How do Nostr communities work?

Short answer

There is more than one community model. NIP-29 describes relay-based groups where a relay enforces membership and posting rules, while clients display group events. Other approaches have existed, so check what your client and chosen relay actually support. A group can move or fork, but its history and rules depend on relays willing to host and enforce them.

Sources

NIP-29: Relay-based groups.