The brightness of the Xiaomi smartphone's display can be changed in the quick settings (status bar) or in the preferences.

Note: The display is one of the largest consumers of battery power. 100% brightness or illumination puts more strain on the battery. Automatic brightness can also be activated for optimal battery life.

Android 10 (MIUI 12.0)
Step 1:  Open the  Settings
Step 2:  Tap on  Display
Step 3:  Tap on  Brightness level
Step 4:  Swipe  left  or  right  to adjust the  brightness level
  1. Open the Settings
  2. Tap on Display
  3. Tap on Brightness level
  4. Swipe left or right to adjust the brightness level

Xiaomi Instructions