Are passkeys replacing passwords, 2FA, or both?
What if I created a passkey on some device, lost that device, and my passkeys aren't cloud-backed-up? Would I be able to recover my account, or it's doomed? Or does it depend on how a given website implemented it?
You kind of have to go out of your way to not have your keys backed up. By default, the easiest route is using your android or iphone and both of them back the keys up using icloud Keychain or google password manager. 1Password, bitwarden, all support syncing. Chrome will allow saving it to icloud or your google account. Keepass can be manually synced. Windows is adding sync in the next update for windows hello. List goes on.
The other thing is that multiple keys can be created. Easiest way to see this in action is google's account security settings. Log in (if you have an account), hit create passkey, see your options and play around with them. You'll also see you can add a hardware security key too, which isn't nothing new but if you have one that's another key that doesn't rely on a mobile device!
If all else fails, the usual account recovery process applies. Much like it would if you forgot your password.