There's a whole set of fallbacks built in to the standard, including Bluetooth, local network connections, and going via a relay server. All of them eventually end up with your device signing something and handing that back to the browser on the other device to complete the authentication flow.

AFAIK, the "scan this QR code" method of signing in with a passkey on a phone on a device w/o the passkey requires Bluetooth. There's some type of handshaking that goes on in order for you to prove you're in physical proximity of the device you are logging in on, to prevent phishing attacks.

I cannot speak to how accurate your description is, but this description sounds like there are multiple weak points and multiple attack vectors that open this up to increased risk of compromise, undermining the very security stance it's supposed to provide.