SIMPLE FIX!!! I had been having this issue on my iPhone 6s for years (yes i still have an iPhone 6, don’t judge me!)— a clicking/buzzing sound that is extremely apparent when recording videos, but not when recording voice memos. i have finally fixed the issue, leaving this answer here in case it can help anyone else. i would imagine this could help with newer models as well, although microphone placement might be in different places.
i learned that there are 3 mics on the iPhone that are used for different things. when recording videos using the front-facing camera, the mic inside the earpiece speaker is what’s used (the long thin thing next to the front camera). clean that really gently with a clean toothbrush and/or very carefully poke some holes through it with a pin if there is dirt clogging it. do the same to the other mics if you want, located at the bottom of the iPhone and on the back next to the rear camera.
that’s it! no more annoying clicking sound when recording videos. i couldn’t believe it was that simple.
other people can also hear my voice much clearer when i put them on speaker phone during calls.