How files and layer work #526

We need some clear instructions on how QGIS files and layers work. It would be good to have some basic diagram of how files related to projects are organised and listed in the same folder and relative to the QGIS project on the PC. And then, when user wants to add a new layer, how they should do it (I know there is a section on it in the docs, but due to search and indexing, I never manage to find it).

The section can go here: https://merginmaps.com/docs/layer/best-practice/