Hi, RodG1,
According to the Apple Community Specialist, it does not appear that a webcam can be used as an external camera with iMovie.
https://discussions.apple.com/thread/251193166
However, I have found that an iPhone, in conjunction with QuickTimePlayer, can be used as an external camera. See my comments in the thread link below:
https://discussions.apple.com/thread/251298783
I would think that if an iPhone can be used then a webcam might work as long as it is recognized by your Mac. So, I think that you just need to try your webcam and see what happens. Might or might not work. Also, as you mention, you can use the built-in camera to record directly to iMovie.
-- Rich