Display is always dimmed

Since you already troubleshooted with another device, the problem clearly seems to be in the screen or in the components attached to the screen. Brightness is controlled by the ambient light sensor in the screen, it could be that this sensor is damaged and it’s sensing “darkness” so it would dim the brightness of the screen backlight to not blind you while being in the dark.

If you’re up to it you can try to swap those two parts of the screens you have and see if that fixes the problem.

Have a look at step 26 of the iPhone 11 Pro screen repair. Note that this will require you to swap out the whole speaker & sensor assembly of the screens.