How might one create a motherboard?

Answer 1

Making a computer motherboard is not the sort of thing one could do at home. They are quite complex. The board itself has several layers, which allows a compact design and connecting tracks pass both sides and sometimes internally. They use Surface Mounted Devices (SMD) which generally require precise jigging and robot assembly. The soldering is done by floating the board on molten solder.
The idea of a mother board could be used in other projects. Core components can be mounted on the board, then have lots of multiway sockets where other boards, with peripheral inputs and outputs, can be added and/or changed.