Welcome to the Apple Support Communities!
I understand you would like to know a little more about the brightness controls for your iPhone. The auto-brightness feature is enabled by default, so you can turn that feature off if you don't want to have to continually adjust the settings. The resource below provides some additional features, such as True Tone, that can be adjusted for your display.
Adjust the brightness on your iPhone, iPad, or iPod touch
Enjoy your day!