With Auto Brightness, the screen brightness automatically adjusts to the ambient light conditions, but if Auto Brightness is enabled on the Xiaomi smartphone, it can still be manually adjusted.

Android 10 (MIUI 12.0)
Step 1:  Open the  Settings
Step 2:  Tap on  Display
Step 3:  Tap on  Brightness level
Step 4:  Enable or disable  Automatic brightness
  1. Open the Settings
  2. Tap on Display
  3. Tap on Brightness level
  4. Enable or disable Automatic brightness

Xiaomi Instructions