What do you mean by underexposed photos?

Take correct, overexposed and underexposed photos.

If the middle part of the above photo is exposed correctly,

Overexposure means that the photos are overexposed and the left side of the picture is overexposed.

Underexposure is dark, as shown on the right.

Principle: Generally speaking, if you are not sure, you would rather be underexposed than overexposed, because overexposed photos are generally less likely to be remedied later. Because the details are lost, it's no use using the computer to dim them. Underexposure can be easier. If the underexposure is not serious, you can usually brighten them with a computer.

The camera can control the exposure with shutter, aperture and ISO.