We wrote a long post on Passkeys, in particular how they are implemented by Apple[0] that might be interesting.
Technically a Passkey is just a multi-device FIDO credential that is compatible with WebAuthn (which is an official W3C and FIDO spec).
However, vendors implementations of Passkeys/FIDO credentials differ quite widely. The Apple implementation of Passkeys, as an example, doesn't provide attestation information which reduces the ability to do device verification. Similarly, even though it's not technically part of Passkeys, Apple removed the possibility to create device-bound WebAuthn keys which significantly weakens the security guarantees you'd normally get with WebAuthn.