If we want to record only the internal sound of the mobile phone and not the external sound, we just need to press the on-screen recording icon (that is, 3DTouch) in the control center. That is to say, the following red microphone picture appears, and we just need to turn off the microphone audio.

When the microphone audio is turned off, the picture will turn black. At this point, you click to start recording. The mobile phone only records the built-in sound, and there is no external sound at all, and the sound quality is very pure.