When did the industry end up being centered in Detroit anyway and why?
A combination of geography and luck. First of all, Henry Ford and Ransom Olds lived there. As for the geography, iron ore was imported from the Mesabi Range in Minnesota, and there was ample timber in Michigan itself. (Early car frames were made out of wood.) Rail and water routes allowed easy shipping of cars to cities like Chicago and New York City. On top of this, Detroit already hosted heavy industry like machine shops and stove works.