Questions about PCB design
Designing a PCB board computer that is very similar to a
raspberry pi or an Orange Pi. It needs to have no ports aside
from Ethernet. It should plug directly into an electrical
outlet and have another electrical outlet on the other
side of it that can work just like a normal port outlet.
The board need to have a switch that cuts off the power
to the other power outlet but the computer is still powered.
Is it expensive to manufacture?
First you can buy a few mains networking widgets with through sockets, something like the BT Broadband Extender Flex 1000 kit. Then you pull one apart, and find out what it is built from. You operate another, to see what it does. You hack a third, install linux on it, add a relay for the mains output etc. That's called research. Thus informed, you can then go to a design house with real specifications and a stab at a bill of materials.