How do I stop brightness on my iPhone 13 from adjusting automatically?

You must be referring to the TrueTone on your iPhone. To turn it off on your iPhone, open the Control Center, select brightness and turn TrueTone off. If you turn off True Tone, the display keeps color and intensity constant, regardless of changes in the ambient light.

Learn about ⇢ Adjust the brightness and color temperature on your iPhone or iPad - Apple Support

-Zachyy ✌️

Open Settings ➢ Display & Brightness.

Scroll down⇣ and look for True Tone as well as Night Shift.

Also: Settings ➢ Accessibility ➢ Display & Text Sizes ➢ Auto-Brightness.

Good Luck! 👋🏼😉

No worries, that should fix the automatic brightness. Have a wonderful day!

Check Settings>Accessibility>Auto-Brightness