They are giving you javascript which executes on your machine in a browser you control. You can override those APIs do do whatever you want. In my teens, this would probably have served up goatse. Now it's likely to get a picture of a stick figure labeled with my name. They might get upset about defeating the webcam. They probably won't hire you. Whether you can afford to stand up to surveillance as the norm depends on your personal views and financial situation.

Should be similar, but you might need to set up some hacks to spoof the browser in thinking it's the screen.

https://testrtc.com/manipulating-getusermedia-available-devi...

No, but other client-side measures could.