Mostr is a bridge between Mastodon servers and Nostr. A Mastodon server generates Nostr secret keys for its users from the users’ Mastodon names and a secret key specific to the server. The Mastodon users don’t even know their own nsecs. Posts to the Mastodon server are forwarded to Nostr relays (the server administrator’s choice, I believe), and Nostr users’ notes are (sometimes) forwarded to the Mastodon server’s users.