To create curved parts, an operator would roll a piece of metal between two hard wheels on a machine called an English Wheel. The wheels apply significant pressure, and make the metal become thinner where you roll it. The same volume of metal is still present, so to become thinner it must spread out. If you expand the center of a piece, but leave the perimeter the same, then it will become domed.

If you'd like to see the machine in operation, here's a video by Ron Covell, a masterfully skilled sheet metal craftsman: https://www.youtube.com/watch?v=omRlIBONJAM

A craftsman can carefully shape the metal to whatever shape was desired. But accurate? Who said anything about accurate? It's likely that everyone's paneling was of slightly different curvatures. They'd machine the mounting holes after the fact, so those would all match and be replaceable, but why would anyone care (how would anyone know?) if the curvature of the fender was a few millimeters different from another fender?