Fewer and fewer. A major goal of engineering is to design systems that can be maintained with the least possible training. For instance, modern auto mechanics d

Fewer and fewer. A major goal of engineering is to design systems that can be maintained with the least possible training. For instance, modern auto mechanics don't need to know that much about car systems because they can read out fault codes and replace whole assemblies. Maintaining pre-WWII cars required a lot more experience. It's only in low-volume applications that there aren't manuals and procedures for everything. Factory robotics is one example I know about. Installing systems and getting them to work reliably is both hands-on and deeply technically challenging.