This part is the floor, and it's the part where I extended the faces along the normals. When I finished, it looked like this. Please help.
Just try again. Going along the normals can be a bit finicky, depending where they are pointing and the direction you moved your mouse to. So you can take the lesson from the beginning and try as the teacher does as closely as possible.
But from the looks of it it's all about the bevel modifier. It goes crazy because vertices are very close to each other. Maybe try lowering the bevel amount.