How do Passkeys work?

Depending on what particular level of “how do they work” information sought , here is some reading:

General usage: Use the Passwords app to create, manage, and share passwords and passkeys across Apple devices - Apple Support

Background from the UK national computer security center: https://www.ncsc.gov.uk/pdfs/blog-post/passkeys-promise-simpler-alternative-passwords.pdf

Background from Apple: About the security of passkeys - Apple Support

Passkeys bypass the need for a password by using biometric verification like TouchID or FaceID. Once you set up a passkey with a website you no longer need to remember or enter a user id or password. The website itself must support passkeys of course and not all do yet.

Passkeys are more secure than passwords because they cannot be guessed or stolen by the bad guys. Only your fingerprint or face get you in.

I convert to passkeys whenever possible.