On occasion, the amplifier will be attached at the rear of the head unit; however, most audio systems place the amplifier in some other hidden location in order to dissipate the heat.
Sound energy is measured in wavelengths that are known as hertz, which go from 20 to 20,000 cycles. A cycle being the distance from the top of one wavelength to the top of the next wavelength.
Depending on the budget that you have to spend on a car audio system, you can assemble a unit yourself or buy a unit that is an all in one system.