Not the ones for kids. The ones for grown ups.
From what little research I’ve done, it seems like most automotive seats are a metal frame, maybe some metal springs, and some polyurethane (or similar) foam, with a cloth or leather covering. Let’s ignore the fancy air conditioned or massaging variety for now, and look at a basic car seat:
Unfortunately this episode of How It’s Made is more like How It’s Assembled, but hey, it’s a start. In the video, the metal frame appears to be made from stamped metal. I vaguely remember seeing other cars (or at least burnt out husks of cars) that use tubular steel frames. I suspect these are less common.
Now let’s look at something a bit more wild - like the seats from the Porsche concept shown off a little while ago:
(Image taken from here):
Like pretty much every other concept car out there, there’s no way seats like that would ever make it to production. But why? What does it take to allow a car seat to be mass produced? Does it mostly rely on crash test ratings? Surely there must be other car seat related safety regulations. For automotive racing, there are regulations. I remember hearing that (supposedly) if you want to run at Bonneville in the faster speeds, you can only have a metal seat. Something about fire safety, I’m guessing.
But let’s say a person was building a car - a kit car. What would dictate that a given seat is safe or not? Could a person build their own car seat (supposing it at least looked safe), and have it pass inspection? Obviously yo do want to be reasonable and have at least some thought for safety. I don’t think anyone is going to want to ride around on a carved oak stump, even if it is bolted down.
I imagine a lot of it might depend on the state/province where it is being inspected. Here in Alberta, things seem pretty lax with regards to what can be on the road. I suspect it’d be a different story in California.
(This is the sort of random things that I think about while preparing to fall asleep at night.)