Any ideas on how this skybox in the udk tech temo was made ?

Here it is for those who haven’t seen it.

First are those buildings just 3d geometry in front of the skybox ? Also are the flying cars just more animated 3d geometry in front of the buildings ?

I’m still very new to game design . So sorry if this seems like a stupid question .

Skyscrapers in the back are painted on skybox, small buildings in front at 00:08 and high at the right are simple 3d meshes, flying cars appear to be just 2d sprites, when he flies through the scene they appear flat.