I still don't understand how the QR code flow works with using/creating passkeys on other devices. It seems it uses a bluetooth connection to your phone, but I can't find much documentation about the exact protocol
It's called hybrid transport, and the old name for it was caBLE. Yubico has a nice explainer here: https://developers.yubico.com/WebAuthn/Concepts/Hybrid_Flows... It's not fundamentally different than other transports for other roaming authenticators like a USB key; the QR code sets up a BLE connection between both devices and presents the authenticator with a challenge for it to sign and log in the originating device.