I think they buy the engine, and modify it a bit to their purposes. Tires, and wheels, too. Maybe the batteries for the EV, too. I think they build everything else, such as the frame, drive train, body, seats, etc.
The electronics are pretty cool. They have a feed back system that monitors things on the car just like real Forumla cars, though I’m sure it’s way more primitive. My son is a straight A student, but he says most of the kids here are way smarter and more accomplished than he is(partly because they spent their teens on mechanical and robotic teams while he was trying to hit 85 MPH on his fastball), but many get lower marks than he does. They essentially blow off classes because they find them uninteresting compared to the club. Employers don't seem to mind... most of these kids get multiple, very attractive, job offers both during, and after school.
My wife is the one who would be going to the WI air fair, specifically for the Ultralights, though she is into anything that flies. As a matter of fact, I had to divert the whole family and go there on a summer vacation, drop her off, takes the kids to DQ, sit in the car forever, then go find her because she forgot about the time. Her dream is to get a plane... it used to be Ultralights, but she got turned off by the company a bit, and there are other things out there that are similar.
Me, i'm a wood guy, and a very retro one at that. Also some painting... i like what they did, but i could do something WAY cooler, though i'd have to research the materials to make sure that the finished job could withstand the heat and moisture(they had to stop the last test at one point when the car temps got to be over 200 F - the driver's body temp was over 100 F!). I did dabble is electronics while building guitars and fixing tube amps, but in a very unschooled way. I do get some of what the MechE guys are doing, especially building the structures, but designing and building PCB's that control functions in the car is way beyond me.