I've plugged in a USB camera to computer and have opened the Camera app. How do we switch from the built-in webcam to the USB camera?
Unintuitively, the icon in the upper left that looks like it's meant to switch between your front and back camera will also cycle through your other attached cameras. At least, this worked for me on a Surface Pro 4 (front and rear camera) with a USB Microsoft LifeCam attached. The Camera application's version is listed as 2018.227.30.1000.
Open the Camera app.
Swipe in from the bottom edge to see the app commands. (If you're using a mouse, right-click within the app.)
Tap or click the Change camera button.