Back in the day, designers who wanted to create a Hot Wheels car based on the real thing would have to get the measurements of the real car and then scale it down. Sometimes, like when Harry Bradley "obtained" the plans for the new Corvette, the measurements were already provided for the designers. But sometimes it was a matter taking a tape measure out to the parking lot and doing it the old-fashioned way.
In the digital age, however, no one measures anything in real life. Nor do they have to sneak in the unlocked cafeteria door. Since 2004, auto manufacturers have just sent CAD files of their cars to the Hot Wheels designers. Even with this technological advance, designers still have a difficult job to do. It turns out that when you hit a real-life car with a shrink ray, it looks weird. The proportions get sort-of funky. The little version is too skinny, and the wheels are in the wrong place. The designers start by adjusting the wheelbase to make the die-cast car look right before adjusting everything else.
Only about half of the cars Hot Wheels puts out in a year are replicas, though. The other half spring fully formed from the designers' imaginations. These are sketched out either on paper or using software to begin with, and then they're transferred — like every other image in the known universe — to Photoshop, where the design can be manipulated. Lately, because we live in the future, Hot Wheels designers have been able to send their designs to a 3D printer for prototyping so they can see what it looks like at the right size and in the real world. Once the designer has created the car he imagined, he hand-finishes the body to make sure it's exactly right.
At this point, the car is sent to the factory to make the die from which it will be cast. That's why they're called "die-cast" cars. Basically, the prototype is used to make a mold, or die, which is then filled with molten metal under high pressure. Once the die is cast, it's also hand-finished to smooth out any imperfections before the car goes into full production.