I've always been curious about the "myth" that: A majority of cars have a stock amp burried somewhere in their dash;

No, this is not true. Such cars exist, but they are in the minority, not the majority. Usually they are the top-of-the-line cars with the "premium" factory sound systems, but even the premium cars don't always have extra amps. It depends entirely on the manufacturer and the system.

Most of the time, the amplifier is built in to the head unit itself, like most aftermarket head units. Sometimes, the amplification is in an outboard amp. Sometimes, the outboard amp only powers some of the speakers while the others are driven by the head unit.