Gun frames can be made out of plastic or aluminum, and there are fixtures for benchtop CNC machines that can be used to make them. This is not nearly as complicated as you make it sound. I think Cody Wilson was basically selling a turnkey solution for that, maybe still is.

AFAIK they claim to still be selling general purpose CNC machines that aren't marketed as being for firearms... but only take the money and ghost customers without actually delivering anything.