iPhone Xr Screen Keeps Dimming

There are 4 reasons the screen may dim on its own:

1. The screen will dim a few seconds before it locks if you are not actively using the phone
2. There is a feature called “Attention Aware” that will dim the screen if you are not looking at it - you can disable this feature in Settings/Face ID & Passcode/Attention Aware Features, and also Settings/Accessibility/Face ID & Attention (Same setting, different ways to access it)
3. Auto Brightness will adjust the screen brightness based on ambient light levels - Settings/Accessibility/Display & Text Size/Auto-Brightness
4. If the phone’s temperature gets too high the screen will dim to lower the heat load on the phone.

You may have a separate battery issue, but that would not affect the display dimming.

To check the battery go to Settings/Battery/Battery Health. Read what it says, and check the Maximum Capacity. If the capacity is under 80% you probably should replace the battery. Likewise if the page displays a note that the battery performance is degraded.