My best guess is that when this is needed a liquid copper if formed, not like super heated copper, but a binary chemical that forms copper, just drill the hole, and let the chemicals do the rest, either that or electroplating, which is probably how copper clads are made in the first place.
I mean I know how to give something a copper plating, only problem is it has to be able to conduct electricity, which the physical board can't.
I dunno how they're made all I know is that they work heh. Just like tanks...