To add, silicon can, and obviously is inspected by manufacturer using optical and electronic microscopes. There are companies specialized in reverse engineering

To add, silicon can, and obviously is inspected by manufacturer using optical and electronic microscopes. There are companies specialized in reverse engineering chips or verifying existing components. Even hobbyists can grind down chips and figure out what the circuitry does, using nothing more than good optics and a digital camera plus freely available software to stitch the images together and start analyzing traces from the picture. Sure, this stuff gets harder with modern technology, but it would be ridiculous to assume that manufacturers blindly click together chips and hope for the best because they can't inspect their work.