Check out this test comparing the screen brightness of the iPhone 13 vs iPhone 12.
To perform this test we disabled Auto Brightness and True Tone in iOS and set display brightness to 100%.
The iPhone 13 performed better than the iPhone 12 with a peak measured brightness of 894 nits (cd/m2). We measured the iPhone 12 at 681 nits. For reference, Apple says the iPhone 13 delivers 800 nits max typical brightness and the iPhone 12 is said to deliver 600 nits max typical brightness.