Switch webcams based on which one you're looking at.
This extension allows you to have multiple webcams used for a video call and will switch to whichever webcam you're looking most closely at.
You can change how often the face detection is checked and as well as how long you need to be looking at a new camera for it to switch (to protect against a quick glance).
This extension was uses
And follows the following technique to create a virtual webcam