If I understand correctly, the routed shape of the pickguard lifts away from the body as a separate piece.
But it appears to match exactly to the surrounding wood of the bass body, implying it was cut from that wood.
Assuming the cut didn't go all the way through the body, then the only way I could do this would be to assemble the top, cut the cover, glue the top to the rest of the body, route the cavity and when all is done put the cover on.
Is that how it's done?