It's an open protocol, but more like the www than email. You can port your identity to another host at any time. You can self host if you want. You have complete control over how your chosen feed algorithm aggregates posts from the firehose. If you want to make a competing service to Bluesky you can even do that, and it will still interoperate with all the Bluesky users.
Whereas with Mastodon, when you pick an instance you're essentially picking which benevolent dictator you want mediating your experience. Mastodon is decentralized in the sense that it breaks the platform up into smaller fiefdoms; Bluesky is decentralized in the sense that you retain control over your own experience.
Some recommended reading on Bluesky/atproto:
https://bsky.app/profile/laurenshof.online/post/3la5j3qgqvo2...
https://fediversereport.com/a-conceptual-model-of-atproto-an...