In order to turn automatic display brightness changing off, novice iPhone users usually go to the Display & Brightness section of iPhone’s Settings. However, the auto brightness switch isn’t there. It’s actually in the Accessibility section since iOS 11 version.
In the current iOS 13 as of March 2020, it’s here:
Settings > Accessibility > Display & Text Size > Auto-Brightness (switch)
By default, auto brightness on iPhones is on. Switching it off can help you with extending the battery life – by preventing display to go brighter in bright environments. But also, in darker environments, where relatively low brightness can be usable, the display won’t get dimmer if the Auto-Brightness switch is off. You will need to adjust (reduce) the brightness manually if you want longer battery life. That can be done here:
Settings > Display & Brightness > Brightness (slider)
Control Center (swipe down from the upper-right corner of the screen on the iPhone 11, 11 Pro, Pro Max, X, XS, XS Max, or XR; swipe from the bottom up on older iPhone models) > vertical brightness slider.
Of course, you can you use these sliders to set the display to be brighter than it would be with auto brightness on, but that would reduce the battery life.
As a side note, manual change of brightness using the sliders while Auto-Brightness is turned on is only temporary. As soon as the ambient light changes Auto-Brightness will kick in and change the level you previously set.