Hi,
typically, you would need a special animation done by the artists and play that when triggering a special collider next to your car door.
if you want that all programmatically, then you need IK, either using mecanim and then controlling each arm legs and the body itself so that each go and target some predefine positions on the car, or another solution on the asset store like Final IK, but I would not recommand that as it's a LOT of work in both case,.
Bye,
Jean