During my training I realized the curriculum was directing my continued learning after completion of my education towards the purchase of proprietary software solutions. I recommend you look at FreeCAD and LinuxCNC, both excellent open source platforms.

The best way to learn of CAM eg. Powermill in my opiniom is that:

you should take some draw of element,

in first step you should prepare strategy of machining,

in second step you should try make exacly this strategy by CAM

How could I find the appropriate strategy for machining ? Is there a reference by which we could differentiate between different strategies and choose the proper one?

You can find these basic informations in fundamental books about machining. Rough machining and finish machining, allowences, stiffness of machining system and machine tool etc. A lot of knowledge you can get from experience in machining