Nostr clients like Damus, primal, coracle, etc don’t rely on a single server like other apps. Thats part of what makes the nostr protocol different from other social media. The othe significant difference is that your NPUB is your public identity and it remains anonymous until you dox yourself. You can use your identity NPUB/XPUB across multiple clients. Never share your XPUB and never enter your XPUB into an sus app or client.

Any nostr user can host a relay and any client or app can access multiple relays. some relays are private/paid others are public/free. Most relays only return content by users that share directly to the relay so in some cases a relay might host for specific topics or locations.

Going a layer deeper, nostr uses “Event Types” to signal the type of content that is being shared. So again client applications and relays can specialize in different content or work with multiple content Event types.