I created a pass key. It was clearly assigned to my user name account. I'm on a Mac in my iCloud. Obviously we can create mini but how does it work on your site?

A passkey you create on your Mac is stored in iCloud Keychain, tied to your Apple ID. When you visit our site and choose Sign in with Passkey, your device simply uses that stored passkey and logs you in with Touch ID or your Mac password. It’s automatic across your Apple devices, and you don’t need to manage anything manually.
You mentioned “mini” — if you created more than one passkey, your device will simply show you the list and you pick the correct one. Most users only need one.