Nostr is decentrialized, working with public/private keys. there is a very basic message format, messages are sent to "relays" that forward and store messages to other relays. It is up to the relays (anybody can be a relay) to connect to each others, chose a policy what/whose content to forward, whom to grant access, and how long to store and re-broadcast messages.

If you are familiar with the IRC chat system, it is similar to IRC but with JSON messages and the ability to store & resend messages on the servers. Servers have to connect to each other and are free to each have their own policies.