Automatic brightness is a separate feature. You can turn it off here:
Settings → Accessibility → Display & Text Size
At the bottom of that screen is a setting for “Auto-Brightness”. With that turned off, the brightness will not change based on ambient light. You will need to manually adjust brightness.
I recommend leaving this feature turned on. You may disagree, but here’s my experience:
My iPod Touch doesn’t have this feature (no ambient light sensor hardware), so I need to manually dim the display when I’m in a dark room to avoid getting blinded by the screen.
Worse is when I’m outdoors in daylight and the brightness is all the way at the bottom (e.g. because I was last using it at night in bed, in a dark room). The brightness is so low that nothing can be seen at all. I need to find a dark location where I can see the screen in order to access the control center to make it brighter. Likewise for the lock-screen’s keypad.
I would really like auto-brightness on my iPod.