Easiest way (in Unreal, altough my experience is limited) - model your car and keep parts separate, link the different parts that move to helper objects (also linked to a master helper object in hierarchy) and animate those. For example, you'll have a dummy for the door (with correct pivot), animated to rotate and open,…
If you absolutely cannot deal with rigging, you'll need to do the thing Mark Dygert recommended against, and that's setting up the animations in Unity. To learn how to do that, check out the Unity Learn section on their website. I believe some of the beginner tutorials will explain how to set up simple animations. I…
I'm mostly speaking from expereince with Unreal and Blueprints but a skeletal mesh is probably easier to work with in game. Its less assets to juggle and you control animations (open/closed) on that asset instead of setting up a fairly cumbersome system in the engine. Since it's a car you probably want other item to move…