How to use external OTG webcam for video calls/conference/talks?
Android 9 has native support for external cameras, but in order for it to work it should be implemented by the manufacturer, which according to anecdotal reports I saw online isn't done by most or even any of them. I just faced the same problem myself. 

One solution is to patch the Android kernel on your device to turn on the support for the feature at the Hardware Abstraction Level (HAL). People have done it successfully, but you will lose your warranty and it's not easy to figure out.

Another solution is to take an open-source app and tweak it to support UVC cameras using an appropriate library. I've got an estimate from a highly experienced mobile developer that it'd take 40+ hours. There was a feature request made to add UVC-webcam support into Jitsi Meet for Android. You can follow the link and express that you need this feature too, maybe the maintainers will prioritize it.