The PCB is the motherboard, it includes the ARM Processor, all the main components of an ARM Powered device. The PCB designer designs that PCB according to plan to fit a certain amount of functionality into a given price point. They take the ARM SoC and puts it on a compact motherboard design, one that fits in Android tablets, Smartphones and more. In this video you can see some of the young engineers working at http://richtechie.com on the current and future Freescale i.MX51 and i.MX53 designs that this PCB makers customers want to release to the market.
Also watch my other videos in my series "Shenzhen Behind the Scenes":
1: Hongda Factory tour
2: utopiacn, Apad Android Tablet maker
3: Richtechie.com, Freescale i.MX51/53 PCB designer
4: Walking around the Shenzhen smartphone market
5: 3Gnet Factory Tour