Machinist would probably be the people that make the actual parts. They're guided by technical drawings, that specify the range of precision or whatever, as well as just how the thing is supposed to be. I think technical engineers make the drawings? I had to make them for machine-class, it fucking sucked. You need to be so incredibly clear with your lines.
A lot of things need a high amount of precision - semiconductors for example. That also just makes them incredibly difficult to make, which increases costs across the board. It's also just a pain in the ass for a machinist - Why does the cupholder need to be precise to within the 1000th of an inch? No reason.
This whole thing is incredibly dumb.