Is the microphone of Studio Display or MacBook used when the two are connected?

I use Studio Display (SD) monitor as my monitor and connect it to MacBook Pro. When I am in a Teams meeting, people complain that there is echo. Then I plug in EarPods and the problems get solved. My guess is that when I am not using EarPods, two microphones are active: one from MacBook and one from SD. So since two microphones receive voice, it creates an echo. I would have expected that when SD is connected to MacBook, one of the microphones should switch off (logical is that microphone of MacBook is switched off). But apparently Apple did not think about this. So what is the solution? I should add that I use Teams under Parallels. So maybe Parallels does not understand that SD is connected to MacBook. Though this also happened in Google Meet while on macOS.