Hey, thank you! We used a method called sprite stacking, where you layer multiple 2d sprites on top of each other with a slight offset. It gives this weirdly convincing and cute 3d effect. So the car sprite actually looks like this (the first one is the shadow):
We then used an outline shader on the whole stack to enhance the feel of it being just a single sprite. And of course the whole thing had to scale, so the sprite was designed to work with 9-slice scaling (that's why there's no detail in the middle or on the sides).
We also made the buildings and guns with this method, and I really love how the whole thing turned out! I highly recommend checking out sprite stacking if you are interested.