Adaptive brightness uses the ambient light sensor and machine learning to adjust your display brightness automatically. When you change the brightness slider, your phone learns your preferred levels for various lighting environments. Ideally, you’ll need to use the slider less and less.
If Adaptive brightness stops working as desired, you can reset it to retrain the system.

If you prefer to manually adjust brightness, go to Settings > Display > Adaptive brightness and turn Use adaptive brightness off .
If Battery saver is on, the screen dims when the battery gets low.