Loading layers in QGIS involves different steps, which are as follows:
Loading a layer means loading a reference to the layer and its metadata. The layer is not necessarily loaded in memory, but is usually fetched only when data is accessed to be processed or visualized.
Loading and registering a layer are separate steps. A layer can be loaded, processed, and modified before it is visualized, or it can be loaded as temporary data for an algorithm. In this case, it's not necessary that the QGIS framework would be aware of the layer.
iface object has shortcuts to load ...