sure, they are decentralized, but still, there's only one service. You can move to a different "server" but BlueSky is BlueSky....here there's Pixelfed, PeerTube, Misskey, Sharkey, etc. You can't move your data between all of them, but there's a few you can like Mastodon/Pleroma/Sharkey They have this protocol, but it's too new and I don't know if they're actually allowing devs to work with it yet for their own projects.
am I right that nostr does have these different services and has a similar model?
tbh, I have no idea about Nostr. Sorry😅 though I'm sure there's many people here who do. Hopefully you find the answers you're looking for in that regard.