I imagine there is some kind of filler applied with a trowel... I can't imagine how the actual metal in the panels could be straightened??? Am I wrong?

hammer and dolly will do most of it

Heating an shrinking the metal, If you can find a local panel shop that does more than just Remove n replace , go in and ask if you can watch some of the guys in action, its very interesting.

Most inner panel have to be pulled and stretched back into shape in order to fit the outer skin. watch this and learn

It all depends on the severity of the crease and the skill of the tradesman.