-Yes, he uses wheel colliders.Also, are you using the in-built 'wheel collider' for wheels? And how do you make the 3D wheel object rotate like a real wheel, for example if the drive wheels are in air (not touching ground) and player presses accelerator, the wheels will spin in air..
-Wheel colliders use physics, so of course they'll move in the air unless a force stops them.
-and in a real car, wouldn't the wheels move in the air if you give them gas so they'll drive? I dunno. Never actually thought about it until now.