If you want to make something that looks like this, all you really need to do is look up how to do billboarding in Unity to have sprites always face a camera
From there, you just have to treat every object as if it were a normal 3D object
So the game sprites are billboards but how is the other objects? 3d sprites on a plane?
All of the other objects are either 2D sprites, or they're stacks of sprites which are made to look 3D
