QGIS's versatility is due mainly to two reasons. The first is the ability to customize it by adding functions, thanks to its plugin structure. The second is the power of the processing toolbox that can connect different backend algorithms such as GRASS GIS, SAGA, GDAL/OGR, Orfeo Toolbox, OSM Overpass, and many more with dedicated providers.

In this way, for example, we can access all GRASS processing algorithms by using QGIS as the project and presentation manager. Another important ability of the processing toolbox is that it can be used to join together all the backend algorithms, allowing you to connect the best algorithms. For example, we can connect GRASS as a producer for another algorithm that is better developed in another backend such as SAGA. Here, QGIS processing becomes the place where you can add your specific algorithm in a more complex and integrated workflow.

This section is focused on how to code the execution of algorithms that are already...