Loading asks for 78 images (48 icons and 30 enemy sheets) and hands them to useLoader, the one place that knows how the batch is doing. Level then asks for the few it shows by key: no paths repeated and no second download.
The bar is made of untextured quads, which is the whole trick: a loading screen cannot depend on the images it is waiting for. Its fill is anchored to its left edge, so growing it does not move it.