The person in the video is tracing over an imported vector floor plan from a 2D CAD program. You can tell because the line tool snaps to end points. If you floor plan is a raster image, there’s nothing to snap too. You can still trace over it, but it’s all approximate unless you type in numbers for what you draw. In the former case, you need a .dxf or .dwg file as @liamk887 says, and then you need SketchUp Pro to be able to open it.