Their attention to the cost of assembly is - to me - as beautiful as the aesthetic design; they clearly have some amazing mechanical/design/process engineers.
I've also read that they require all parts to be doubly sourced, so that if one source fails, they have a backup. I don't know how common this is, but seems like an interesting point in their process.
[1] He doesn't design them himself, but I couldn't think of a better word.
Secondly, as an Industrial Design graduate, I believe Ive has more of a functional knowledge of subjects like strength of materials. The rest is just experiential from working with industrial engineers and others at Apple. For example, what you hear him saying in the iPhone commercial about the manufacturing is experiential knowledge. His job is to design the form. It's the people under him who give him the best materials based on his requirements.
A good way to understand what the process from top to bottom is, is to hear Jobs explain how they came up with the idea for the iPhone at D8. It starts with an idea that is then handed down to relevant people to think and work on.
Both Jobs and Ive are fantastic at what they do, don't get me wrong. But the truth is they're run as much by other people's ideas as they're run by their own.