We can pre-set the volume for media such as videos, music, or games on the Xiaomi smartphone, but the media volume can be adjusted using the Volume Up/Down buttons when a video is playing, for example.

Android 10 (MIUI 12.0)
Step 1:  Open the  Settings
Step 2:  Tap on  Sound & vibration
Step 3:  Swipe  left  or  right  to set the  media volume
  1. Open the Settings
  2. Tap on Sound & vibration
  3. Swipe left or right to set the media volume

Xiaomi Instructions