Auto dimming

Hey Archerky66!

You need to go to Settings > General > Accessibility > Display & Text Size.

The page you're currently on only changes your iPhone from light mode to dark mode.

Regards.

Hello and welcome to Apple Support Communities, Archerky66!

I understand you want to stop your iPhone 11 from automatically adjusting the brightness. I'd be happy to provide information to help you accomplish this.

Use the steps in the following article: Adjust the brightness on your iPhone, iPad, or iPod touch

"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 > General > Accessibility > Display & Text Size."

Have a great day!