what’s the structure of the house made out of?

Reinforced concrete. It's cheap to create, the materials are quite cheap, it's very strong, and you can make it have any shape.

I never understood why the US makes strctural bits out of wood. I can understand using it on the walls, but it's completely unfit for the structure.

I'd bet most people here claiming their houses are made of bricks have a reinforced concrete structure hidden inside brick molds on the corners.