My best suggestion would be to find a softgoods bag maker in your area. Beg or a shop visit, and learn everything. If there are none in your area, catch a flight to either a softgoods factory or a brand that produces their own bags within their own facilities.
This is pretty much the only way you’ll really learn. You need to see how a bag is made from basic materials into a finished product.
Once you understand those construction/manufacturing processes, you can learn how to design products using those (and eventually start learning other processes like ultrasonic welding, etc).