Passkeys are built on the WebAuthentication (or "WebAuthn") standard, which uses public key cryptography. During account registration, the operating system creates a unique cryptographic key pair to associate with an account for the app or website. These keys are generated by the device, securely and uniquely, for every account.

One of these keys is public, and is stored on the server. This public key is not a secret. The other key is private, and is what is needed to actually sign in. The server never learns what the private key is. On Apple devices with Touch ID or Face ID available, they can be used to authorize use of the passkey, which then authenticates the user to the app or website. No shared secret is transmitted, and the server does not need to protect the public key. 

This makes passkeys very strong, easy to use credentials that are highly phishing-resistant. And platform vendors have worked together within the FIDO Alliance to make sure that passkey implementations are compatible cross-platform and can work on as many devices as possible.

Passwords and recovery codes can still be used to login to your account. However, these can be phished from a user. However, a passkey is not a credential that a user can be tricked into entering into a malicious site.