How do I stop iphone from dimming the brightness?

There are two primary reasons brightness will dim when auto-brightness is off:

1. The phone is in danger of overheating; iOS will dim the screen (which is a major source of heat) to keep the phone within a safe temperature range.
2. There is a feature in Settings/Face ID & Password - Attention Aware Features. This will adjust the brightness and volume depending on whether you are looking at the phone or not. You can turn it off.

Hi amandafrompuertorico,

Thank you for using the Apple Support Communities! If you go to Settings > Display & Brightness is True Tone enabled? If so, does disabling help? As noted in the following article "If you turn off True Tone, the display keeps color and intensity constant, regardless of changes in the ambient light.":

Adjust the brightness and color temperature on your iPhone, iPad, or iPod touch

Hope this helps!

Have a great day.