I haven’t tried it myself, but I believe the OC is referring to the webcam USB mode that Pixels (idk if this is an Android feature or a “Pixel” feature) have. To enable it, connect over USB then you’ll have a notification along the lines of “Charging this device over USB”.
If you click that notification it’ll let you enable Webcam over USB mode, which I assume just causes the phone to act as any other webcam device would.