I need a better camera for ZOOM meetings - on my desktop - what to do?

Yes, you can use external camera as alternate source. I use Logitech 920 which has served me well with 1080 resolution.

I would say, most of the external cameras are USB inferface - you can just plug into the back of the iMac (4 ports available).

In the Zoom, you can choose audio input (mic) and video input (iMac's build-in or external camera).

Here is the link from some education institution regarding "how to" - with different audio/video settings, because COVID-19, indeed, has changed how we interact these days.

https://kb.swtc.edu/82399

See if this helps:

‎EpocCam Webcam app for Mac and PC on the App Store

I don't know for the Logitech c920, actually, I am using a separate external microphone (Samson Meteor) in order to get a better/clearer sound by placing it closer to my mouth.

Unfortunately Zoom update on security has just broken the EpocCam support, for using the iPhone as an external camera... so until they find a workaround, you'd better be cautious before spending some bucks in a 3rd party app...