Cross-device authentication (CDA)
termsecuritycryptography
The flow that lets a passkey held on one device authorise a sign-in on another - the site displays a code, the phone scans it, and the two link over Bluetooth Low Energy using the CTAP hybrid transport.
The common misreading is that this makes the security of your login depend on Bluetooth. It does not: the exchange carries its own cryptography on top, and Bluetooth is used only to establish that the two devices are physically near each other. That proximity requirement is the anti-phishing property - a remote attacker's page cannot be in the room. It is also what makes passkeys usable without syncing, which matters for anyone deliberately keeping credentials device-bound.
Also known as: hybrid transport, qr code sign-in, ctap 2.2, ble proximity