I use Zoom on Linux. My notebook computer has a built-in Mic, which is the "Same as System" default for Zoom. But my external USB webcam has another, better Mic.

So I first had to start up a Zoom "New Meeting" with only myself. This then bought up the main Zoom video window, with a microphone sub-menu now accessible at bottom left as part of the "Mute" icon. Accessing _that_ menu then permitted selecting _which_ microphone to use. I wanted "Camera Analog Mono" here.

I can test all of this by then "Recording" my solo Zoom session, and later playing back the recording to guarantee it all works.

Any luck trying that?

EDIT: Speaking of which, I found/use this excellent USB webcam/Mic combo for CAD$43 in July:
https://www.amazon.ca/gp/product/B088HG6XQ5/