Accessing a USB Camera on a Galaxy Note20

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Hopefully there's a simple answer to this, and hopefully this is a place to find it!

I'd like to be able to switch between the front, back, and a USB-C connected camera for input to streaming apps (notably Facebook Live, and Zoom conferencing, but would like a more general solution). Ideally, video mixing (e.g., picture-in-picture), would be nice.. The primary application is to stream live interviews, site walkarounds, etc., from a gopro or the camera in a pair of AR goggles. E.g., live stream the view from the head-mounted camera, with an inset view of me, from the phone's front camera.

Right now:

- I can almost do it all from a Microsoft Surface - the basic camera app allows selection from front, back, external USB camera. Control of camera selection is through the system settings dialog.

- On a Samsung Galaxy Note 20, I can't seem to find the equivalent of a system-level setting. The basic camera app allows for picture-in-picture still & video from the front/back cameras - but doesn't seem to have a way to select an external camera. Facebook live allows switching front/back, doesn't seem to do picture-in-picture.

Documentation for all this stuff seems hard to find. I've been pouring through the various android developer sites, and device related sites - lots of stuff on low level code interfaces, nothing on what's actually coming across the cables, and mostly out-of-date youtube videos on how to do very specific things with very specific external video switches, and various downloadable apps that don't seem to work very well.

Which leads to the following questions:

- Are there any deeply buried settings - in Android, Samsung Developer Tools, or Facebook, Zoom, Teams - that let one select a usb camera? (Bonus for allowing picture-in-picture.)

- Any recommendations for an Android video switch/mix application that inserts itself between the system video interface, and standard apps?

Hopefully, amongst all the noise, there's a simple solution.

Thanks very much for any assistance anybody can offer.