I create multiple passkeys for the account: an iCloud passkey and a Yubikey passkey.
Both passkeys are independent of each other and know nothing about each other, only the website knows that both are mapped to the same account.
It's like setting up multiple API keys for a service. Passwords are usually restricted to just one, but passkeys are (generally) allowed to have multiple backups.
When I go to login to Google, for example, it will prompt for a passkey. Currently, in Safari, it will prompt for biometrics on my iCloud passkey. I can either give it my fingerprint/face, or hit the "More options" button, which in the current list allows either 1) insertion of a USB security key, or 2) presenting a QR code that a phone device can scan, allowing use of the passkeys on the phone.
(All very good questions, BTW!)