useLoadGltf hands the model back at once, still loading, and createModel puts it in the scene on the same frame. It appears by itself when the file arrives: no loading screen, no callback, nothing to wait on.
Both shapes of the format are read, and you do not have to know which you have. A .gltf keeps its numbers and its picture in files beside it; a .glb is all of it in one. The file is asked, not its name.