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.
Tip
The iface object has shortcuts to load raster and vector layers in a single step instead of loading and registering them via separated steps.
Every layer type is managed by a provider manager. QGIS has some internal implemented providers, but most of them are external libraries. The list of...