Nostr gets rid of the notion of servers and admins. At a high level everyone on nostr owns their own account (no central instance). When you want to post something you send your content to a list of relays you choose.

Other people can choose what relays they want to subscribe to.

Relays can block people from subscribing or posting.

Everything is cryptographically secured so there is no way for someone to pretend to be you.

Lemmy is different where the instance admin has complete control. Admins can post as you and users cannot easily migrate to a different server.