Jewelry designers belong to the category of arts and crafts design. As long as you are a design major, you must have a sketch foundation. If you want to learn, you must start from the basics. You can choose a basic class, and the teacher will start with the basics. You can learn as long as you work hard. It will take years for those designers to finish painting so quickly, so it takes enough effort to become a good jewelry designer.
I used to have a senior buyer in Pengzhong. They have a design class, but I'm not sure about the specific learning content. I suggest you consult their teacher directly.