No, the Unreal Engine is really simple, actually. Everything came from pre-existing assets. The buildings were just multicolored shapes. The coins were made by flattening cylinders and using a gold material, then adding a light source to them. All coding is reduced to blocks inserted in blueprints with interlocking pieces. Here's a picture of one of my blueprints from this game:
Specifically, this is the coin gathering blueprint. This is the code that lets the game know that you've touched a coin, and now it's time to count and delete it. This is how you basically code everything in the Unreal Engine. I gave the coins their peculiar rotation characteristic because all coins just spin like tops and I wanted to be different. Childish, I know, but I thought it looked cool.
The robot is actually (obviously when think about), is actually the free base model human skeleton available in the Unreal Asset Store. I tried to mix things up by adding some hidden areas, too.
No, the doors don't open. If there's a block path, you can just push right through or find a way around.