my screen goes dark too quickly

It could be one of two reasons, here's how to fix it.

Settings > Display and Brightness > Make sure the Automatic Box isn't ticked (this means your set brightness level will NOT change)

In the same section click on "Auto Lock" and change that to a time that suits you, it is generally set at 30 seconds but the phone can dim after that time so you might want to make it slightly longer. This means if you're looking at something the display won't dim.

Settings-display & Brightness-autolock

change the setting.

also :

Adjust the screen brightness automatically

iPhone adjusts the screen brightness for current light conditions using the built-in ambient light sensor.

1. Go to Settings  > Accessibility.
2. Tap Display & Text Size, then turn on Auto-Brightness.

Make sure it’s off.