They use a wafer tester that literally has a tiny bed-of-nails array on it that contacts the bumps for a given die on the wafer, and can move over the wafer in

They use a wafer tester that literally has a tiny bed-of-nails array on it that contacts the bumps for a given die on the wafer, and can move over the wafer in x and y. It provides power and test signals, although power is typically quite limited (<100A of current or so). The good die will then go on to get packaged and further tested before being assembled onto a PCB