There is a lot more to it than meets the eye. You must know power electronics, computer architecture, logic, interface, and some sort of software "glue" to make it all work as one. Each interface has it's own standard.... there is not just one person that designs the mobo - there is a team. When they are happy with the design it moves on to the pcb layout. The pcb is 4, 6, 8, 10.... layer board depending on how complex/dense the design is. Grounding, decoupling, noise, coupling, trace impedance.... all have to be considered. I can go on - I think you get the idea - you will not be putting out a mobo after you've gone through a simple EE degree.