I completely missed the car design feature when I rode TT as a single rider a few years ago. I wasn't even aware it was a thing. So what do you do and what does it mean that the design is saved to your MB?
You use a computer screen to work with different aspects of the car from profile, tire style, paint job, accessories, etc. Each option you choose will effect the cars performance, speed, stability, economy, etc. Whether you design while in line or at the kiosk near the gift shop the choices you made will be saved to your MB (magic band), then just before you get into the car, you will tap your magic band again and your car will load into your ride experience. Now your choices won't change the feel of your actual ride, as 5 other people in the car have chosen different things, but you will get updates as the ride progresses how your car performance compares to the 5 others in your car (a rating system). As you exit the ride, on your right hand side is a huge display screen with several MB tap points, stop here and tap your MB and you'll see some composite scores and how your car compares to all the others that have gone through the ride that day. Once a car is on your MB, you can use it over and over again, or take time to design another and see if you can change your overall performance.
If you design while in line, you are timed, and as others have said the single rider line doesn't allow as many options to effect the performance. But, the kiosks near the gift shop are not timed. So, the wait there can be long. I've been there when it's empty, but when there's just a few people it can be a wait, as each person needs to take time to create. If you have indecisive people ahead of you, you could be there awhile.