Printed circuit boards (PCB) are used in every electronic device. The PCBs, before they show the circuit itself on the board, they contain a layer of copper deposited on them. This is later removed – etched, to be exact – in the unwanted areas, and become the “connecting wires” within the electronic circuit. These days, milling machines are used to remove the copper accurately so that human errors are minimal.
Of course, if the design itself is flawed, no machine can compensate for that kind of human error.
Before milling machines were invented or, before it was affordable to use milling machines for PCB etching, the process was done using chemicals. It is still done in that way, but not extensively, as the process has a tendency to remove all of the copper if not done carefully.