Upon placing a car, automatically puts player in it with fuel. Upon player exiting car, despawns car and puts it back in player inventory, along with any stuff in the car.
1 - If you put any quantity of items from your inventory into the car it will give you double back when you leave the car.
Example: https://streamable.com/pdkfc
2 -If you put any amount of ammo in the ammo slot it will disappear when you leave the car.
Edit: Sorry for the video not being available, i'm not sure why it was removed.