license

I am confused about this SQL license for microsoft,

can someone give me an indepth explanation of how the license works and it should be based on

1. The minimum license that can be purchased and that minimum license, is t for one sql server or can be given to other sql servers when it comes to the 2012 and the core license as well

Please share every indepth information you have with the sql license for both 2012 and 2016 because I have issues with the cals and the core license I know the core license doesn't have cals but vm is involved here