Looking at the technical specifications of some random MacBook Pro computers, none say that there’s a “stereo microphone”. Instead, they reference “dual microphones” “or a “3 mic array”. Personally, what I have found is that these microphones are not up to the task of anything beyond a quick voice recording, dictation, or the occasional FaceTime/Skype video chat session. Even then, I’ve found it necessary to use a proper external microphone. My feeling is that it’s not really a stereo mic, but mono microphones that are digitally made stereo.
Many years back I was tasked with coming up with a way to record a live presentation as it was streamed from a camera attached to a MacBook. They were using the internal mic to pick up the sounds. The loud speakers were in non-eqiadistant locations to the Mac so I was concerned with the how the sound would come out since one speaker was obviously closer to the Mac than the other. Well, the sound was bad but it wasn’t stereo - both channels were identically bad. What I found that in order to get a proper, quality stereo recording, I needed to utilize a mixer and obtain the audio directly from audio gear as opposed to letting the internal microphone do the work.
Bottom line, I would avoid using the internal mic if you can - it’s more for convenience than quality. Use a USB mic or if using more professional gear, a USB mixer so you can manage and manipulate the sound levels properly.