Aerospace vehicles are designed in a hierarchy with a whole lot of individuals specializing in one piece of one subsystem, but by the nature of aircraft everything is very codependent. You can't just design thermo, avionics, structures, aero, payload, etc. in a vacuum; the tiniest details of one subsystem have reverberating effects in all of the others.

At the top you usually have a set of old guys (you might have read Skunk Works, those old guys are Kelly Johnson and Ben Rich in that book) but in this case Musk and likely a few others... a set of people who have to know a little bit about everything and enough to drive the important piece of engineering of the moment towards a successful project.