> A passkey is just a thing that authenticates with FIDO2 (or is it WebAuthn?), I believe.

Not quite. First of all, passkey is not a standardized term. But usually it refers to a key that can be used for authentication on its own, not as a 2FA along with a password.

A FIDO2 key can be a passkey, or not, depending on the service or configuration.

FIDO2 and WebAuthn added some fields necessary to make this work "securely", e.g. asking the key to verify the user separately (e.g. a PIN, which serves as a second factor), or asking the key whether it is device bound or roaming, so individual sites/enterprises can enforce their security policies