My passkey is synched with iCloud which has all hardened settings set to on, so if I switch devices (I have) its seamlessly. You can also export Passkeys is my understanding but I have not tried it.

When I'm on non-Apple I can scan a QR code on my phone and it will validate it the same way, using bluetooth to send enough info to the target computer to get me logged on.

Bluetooth is actually only used to establish proximity. Both your phone and the browser connect to an internet relay, which lets the phone essentially pretend to be a USB security key connected to the computer.