iPhones automatically manage mic gain. If you need precise control, consider a USB or Lightning microphone with hardware gain dial.
Even if your system volume is perfect, specific apps like Discord or Zoom have their own "Automatic Gain Control" (AGC) that can override your settings. how to change mic sensitivity
You want your input volume high enough that your voice is clear at a normal speaking volume, but low enough that it doesn’t "clip" (distort) or pick up excessive background noise. iPhones automatically manage mic gain