How about the polishing of plastic molds? How about the salary? What are the prospects?

Look what you did here. If it is an ordinary mold, the salary is about 1500. If it is a larger mirror polishing or other difficult polishing, the salary is far more than that. It's higher than making molds. You don't need a degree, you can learn it in junior high school. You can go and see the places where you recruit apprentices. If you want to do mold design, you must learn mold manufacturing, not polishing. Polishing is not helpful to the design.