Yes, someone actually does build up the pattern to make a female mold from. Usually the mold is RTV, a rubber-like material that has some flexibility so that no relief angles are needed, giving better accuracy. I assume someday in the future patterns might be made from 3D printers, but I know no resin casters who use them yet for patterns. Of course, with the high price of urethane resin, maybe they would forgo the casting and just make the production parts in the printer. However, with the times I have seen quoted for existing 3D printers, that is tying up an expensive piece of capital for a long time.