Hell, with modern stepper motors and extruded aluminum box frame, you don’t even need a 3D printer to build a 3D printer. It would certainly make it easier, but it’s not required. You could manufacture an entire 3D printer using off-the-shelf parts and a raspberry pi (or maybe even an Arduino) to control the motors. It wouldn’t be elegant, and it would require a lot of calibration… But it would be doable if someone were so inclined.