According to Callum, he and other designers typically just start working with bricks. He says they'll have CAD drawings and models on hand for reference. Apparently the difficult part is getting the car, and its details, down to the scale of Lego's plastic people, the minifigures.
We can understand why. With loads of space and thousands of bricks, you can create a highly detailed model, such as the life-size Lego Batmobile at the Detroit auto show. But the itty bitty Speed Champions cars can only use a few. It's a bit like going from a 4K television to standard definition. Fewer pixels, or bricks, and you have much less detail. Check out the video above for more insight into Lego design.