You can start your own communities on your own server, but if moderators keep removing your stuff you may end up in a community by yourself. People generally don’t like unmoderated places. Most places that claim not to do moderation eventually end up doing so because of this. Look at Trump Social for a prime example.
Nostr has a web client and a few apps. Your account is a cryptographic key, basically, which you can use to send messages through any relay. If a relay goes down, all of your messages on that relay disappear, but your messages on other relays will still work and you can still use the same account to keep tweeting (nostring?).