It may sound obvious, but the first job of a great audio system is to be HEARD not SEEN. In this case, the entire system is designed to use empty spaces and the factory radio to make enough great sound to keep up with the 6.2L 411hp truck it lives in.
The stock radio was retained in order to keep it's functions and the stock cosmetics. If you like it, or if you can't change it for some reason, there's no reason not to use the factory radio anymore.
This is the magic piece that allows it to happen. It takes the signal from the stock radio and turns it into a high powered output for the new amps. It also allows for tuning of EQ and crossover.
Modern subwoofers are thin enough that you can fit them in lots of places you couldn't before. This enclosure holds a downward facing 10" woofer. The stock "premium" woofer is a paltry 6.5".
The new speakers hide behind the factory grilles in the doors. You'd never know they were there until you heard them.
Here's a handy trick: If the factory speaker is a 5x7" oval speaker, it can be replaced with a 5.25" mid and 1" tweeter instead for much-improved sound. These are Alpine's Type-X speakers in the front.