Most modern cars have a unibody chassis, meaning the majority of the vehicle is one giant metal assembly.
So you have to be able to repair the existing metal or cut/weld a new panel in. It greatly increases the cost of doing bodywork on modern vehicles, but provides for a lighter safer vehicle.
Depending on the type of dent, a pushed in "ding" with no creases can often be popped out. Any creases or missing/cracked paint often means the panel will need body work. Some smaller dents can be repaired with a body filler, then painted. If it is worse, it may need hammering/stretching/shrinking, then body filler and paint, work best left to professionals.
Another thing to keep in mind - lots of "scratches" in paint on vehicles paint from other things smudged onto the surface, or shallow and don't penetrate the color coat, and can be polished out with a good polishing compound.