One of the big benefits of passkeys is that you can completely remove the ability to log in with a password!
So no password login, but then you can recover your account by adding an additional passkey by receiving an email.
If you want to log into, say, Google, but the passkey flow is the only way in, then you're almost-completely SOL unless you have some way of getting the passkey out of your iCloud keychain and into the keychain of the phone you're setting up.
If you still have your old phone, you can scan the QR code and get in that way. If you don't, then you're completely SOL.