Read up on metal fabrication, general manufacturing and assembly processes, and then build a bunch of stuff using a minimum of custom components and a maximum of third party modules with an eye on the true costs including BOM, design iterations, and assembly. MIT2.008 (Fundamentals of Manufacturing) is not bad. Search Library Genesis for manufacturing / mechanical and go for the textbooks/references. If starting from scratch you can get a long way for personal experience with a home-built wood/polymer router, a drill, low end MCUs and scavenged parts. Even if you want to focus on design, you can gain from either outsourcing all your parts fabrication (this will also force you to learn how to communicate designs effectively) or getting a membership of a shared workshop/makerspace and learning the machines and processes yourself, which will help you to do better designs.
IMHO it's sort of a schizophrenic field. You could start by dissecting the parts lists from factory automation supplier catalogs, but then you'd likely miss custom mechanical component design, be stuck with fat form factors and have no idea about custom control systems. You could go oldschool machinist like timepiece design and repair stuff, but then you'd miss modern mechatronics entirely. You could go kit/hobby projects but then you'd likely miss industrial standard pneumatics, while you'd gain enough electronics insights for custom mechatronic control. Each area's boundaries have their commercial and historical reasons but there's no clear map of the whole landscape that I've seen, and I'm not an authority.
