Then you'll login to your Apple iCloud account using your password.
Passkeys are just passwords that require a password manager; you can't login to a password manager without something outside the password manager, usually a password. (That's why they call it "LassPass" and "1Password"; there's one last password you'll still have to maintain.)
No password manager tries to get you to login with a passkey without setting a password, for precisely that reason. Apple, Google, and Microsoft do invite you to login to their password managers via passkey, because it's convenient and unphishable, but they always also allow you to login via password (or "backup codes", which are just backup passwords).
That's the point: passkeys, and even 2FA, address rare and unlikely threat scenarios, while defeating most common use cases (delegation by sharing credentials), and making people vulnerable to most common threats (like, losing or breaking your phone).
That is not a rare threat model. Many people use iPhones and Windows computers. Fortunately, Apple has released iCloud Passwords which lets you access passwords and passkeys from a Windows computer.
Passkeys are just passwords that require a password manager; you can't login to a password manager without something outside the password manager, usually a password. (That's why they call it "LassPass" and "1Password"; there's one last password you'll still have to maintain.)
No password manager tries to get you to login with a passkey without setting a password, for precisely that reason. Apple, Google, and Microsoft do invite you to login to their password managers via passkey, because it's convenient and unphishable, but they always also allow you to login via password (or "backup codes", which are just backup passwords).