Your Passkeys have to be stored in something, but you don’t have to store them all in the same thing.

If you store them with Microsoft’s Windows Hello, Apple Keychain, or Google Password Manager, all of which are closed source, then you have to trust MS/Apple/Google. However, Keychain is end to end encrypted (according to Apple) and Windows Hello is currently not synced to the cloud, so if you trust those claims, you don’t need to trust that they won’t misuse your data. I don’t know if Google’s offering is end to end encrypted, but I wouldn’t trust it either way.

You can also store Passkeys in a password manager. Bitwarden is open source (though they did recently introduce a proprietary, source available SDK), as is KeepassXC. 1Password isn’t open source but can store Passkeys as well.

And finally, you can store Passkeys in a compatible security key, like the YubiKey 5 series keys, which can each store 100 Passkeys. This makes them basically immune to being stolen. Note that if your primary interest in Passkeys is in the phishing resistance (basically nearly perfect immunity to MitM attacks) then you can get that same benefit by using WebAuthn as a second factor. However, my experience has been that Passkey support is broader.

Revoking keys involves logging into the particular service and revoking them, just like changing your password. There isn’t a centralized way to do it as far as I’m aware. Each Passkey is only used for a single service, after all. However, in the same way that some password managers will offer to automatically change your passwords, they might develop a similar for passkeys.