Where can we find better documention for python QGIS multiprocessing. Especially regarding the multiprocessing of python and QGIS.
Just to add to @PolyGeo's answer that I also could not find any official documentation regarding multiprocessing (if it even exists!). But there is also another method, described in this blog, which uses multithreading in QGIS which might be useful.
Main difference between the two methods are (more of which is discussed here):
Multiprocessing allows multiple processors to simultaneously run separate sets of instructions (threads). A main advantage of this method is that if an error occurs in one process, it will not have an effect on the other processes.
Multithreading allows for specific operations within a single application to be subdivided further into individual threads. The main advantage of this method is that each of these threads can be run in parallel but due care must be taken as if an error occurs in a single thread, the whole operation could crash.
If there is no QGIS official documentation available for multiprocessing QGIS using Python then I recommend reviewing earlier Q&As here that have tags
There are two with answers that would seem relevant to what you are trying to do: