What part contains the mic?

You are going to have to be more specific; there are three microphones on the iPhone 12 Pro model, depending on what you're doing.

The one primarily used on phone calls is located at the bottom of the phone and is mounted on the charging port assembly.

A second microphone is used when you're on speakerphone or using the selfie camera and is located on the earpiece speaker assembly mounted on the display.

The third one is on the rear side of the phone in the middle of the cameras, and is mounted on the flash assembly.

In each of these images the microphone can be seen as a small gold-colored rectangle.

You should be aware that replacing the earpiece speaker assembly will cause you to lose Face ID. There should be no issue replacing any of the other microphones.