Im assuming by "get the option to design from ground up." you mean design the rest of the car the same way you do an engine?
The car designer will be very similar to the engine designer, picking a chasis, adjusting the size, choosing engine & engine placment, gearbox, suspension, styling it etc.
Business wise, im not really sure what to tell you about it.
You'll build factories, research tech, setup production, run marketing, ship cars around etc.
Here is an over view of how you'll interact with the game
World Map - A view of the whole world, From here you'll be able to see ships transporting your cars and select regions, view world information etc.
Region view - Basically just a more zoomed world view looking at one region.
Plot view - a view of your factory, complete with buildings, cars driving out of the factory and being shipped etc.
Specific views - like the engine designer or car designer, or the factory screen, marketing screen etc.
I'm not sure I've explained that very well, but there you go