If you pop open a computer case you'll find that the motherboard doesn't do much powering outside of ram, cpu, graphics and expansion slots. The power supply is connected directly to most things including hard drives, optical drives, good graphics cards, etc. Thinking like this helps a lot when designing a robot.
The equivalent to the motherboard on a robot would be your microcontroller which is basically your ram, cpu, and long term storage. (Depending on which make and model you get there are extra features but consider them all those other chips on a motherboard). Things such as sensors, motors, and displays are all powered separately. Only the input/output is connected to the microcontroller.
For further information pop open what you're reading this on and look at what connects to where.