If you haven't already solved the problem, here are the things to check on Mac:

MacOS > Apple Icon > System Preferences > Sound > Input:

This should show you a list of the available microphones. Select the one that you want to get the mic sound. Speak into it. There should be a meter on the screen that says "input level:" and it should show your speech occurring on the meter. Adjust its "Input Volume" to the desired level. (You might have to readjust this after being in an actual Zoom conversation and the participants complain that it's too loud or too quiet.)

MacOS > Menu bar > Input/Output selector:

At the top of the screen at the right-end of the menu bar, near the clock, should be the "speaker" icon. Click on it. It should drop down a list of objects named "Output Device". Select USB headset so that it has a check mark next to it. (Even though it says "Output Device" I've found that sometimes this affects which mic I'm using as well.)

Zoom application > User icon > Settings > Audio

Within the Zoom application's own settings is also a separate screen for selecting the speaker and microphone, with their own adjustments. One of the options for them is "Same As System", you can select that and it will use whatever you've set up in the earlier steps. Or you can pick other objects and settings which are deliberately different from the system.

After that, as Mark said, test it by "recording" a zoom session with just yourself. I've never done this but I believe Mark when he says that it works.