iPhone brightness dim by playing games

Your screen will dim if the phone gets too hot to operate within its temperature specifications; try moving to a cooler location. This can happen if you are playing video-intensive games, especially if the phone is plugged in at the time.

Hello tanishq64,

You can find a couple of ways to adjust the brightness outlined here: Adjust the brightness and color temperature on your iPhone, iPad, or iPod touch

Just to confirm, these steps from the same article are how you turn off auto-brightness to stop the screen from changing due to surrounding conditions:

About brightness levels

iOS devices use an ambient light sensor to adjust brightness levels based on the light conditions around you. The sensor lowers brightness in dark locations and raises brightness in light locations. The auto-brightness feature is on by default.

When auto-brightness is on, you'll notice that the brightness slider on your device moves according to changing light conditions.

You can turn auto-brightness on or off in Settings > Accessibility > Display & Text Size. To reset the auto-brightness settings, turn off auto-brightness and then turn it back on. 

If you have followed these steps, and the brightness is still changing, we would suggest reaching out to Apple Support using this link: Get Support

Have a good day.