Ben Einstein, Founding Partner of Bolt (a hardware startup investor and accelerator), has written a terrific four-part post in 2014 that is still applicable today. It outline what you need to know to build a hardware business.
Elaine Chen has a free book "Bringing a Hardware Product to Market" that provides a deep dive into the engineering and manufacturing phases of hardware product development. It is quite old (first published in 2014) so use your judgement. Much has changed and you can do things in Shenzhen that were never possible before. That said, the general mental model of engineering phases and the like is still more or less the same. Check it out here.
Special thanks: Martin Trust Centre, MIT
Edit: Javier Rojas, 15/04/20