I'm not 100% sure. Recently Scawen implemented the "CPW" system which checks for mods, and I'm pretty sure this checks for VOB mods. However if your VOB mod has a "check value" that is the same as the normal car then it gets past the check, although maybe this means that it doesn't affect the collision physics. I'm no modder so I'm not sure
The cars you've seen on youtube are known as "VOB mods" where the VOB file of the car (which details what it looks like) has been edited to make them look like these other cars - they are the same as the standard LFS cars underneath. It's not allowed to discuss VOB mods on the forum as they're modified content but they are freely downloadable from a quick google search. Be careful that any mods you do use don't affect the collision detection system, which means you'll crash even though no-one has hit you.