how is the metal treated that is used to make cars?

Did you mean after the car is made? If so,

In the factory, car bodies have chemicals, typically phosphates, applied often using electrically charged systems so that layers of protection are added to the metal body. Some firms galvanize part or all of their car bodies. This is done before the primer coat of paint is applied. If a car is body-on-frame, then the frame (chassis) and its attachments must also be rustproofed. Paint is the final part of the rustproofing barrier between the body shell (apart from on the underside) and the atmosphere. On the underside an underseal, a rubberised or PVC-based coating is sprayed on. These products will be breached eventually and can lead to unseen corrosion that spreads underneath the underseal. 1960s and '70s rubberised underseal can become brittle on older cars and is particularly liable to this.