Thanks Federico, I wish that link (which I have visited many times already) gave me a better understanding. Maybe someone here can help.
Do I manually create a new version folder (say ...\sam\1.6.5) locally and then grab the files from Github and place them underneath this path? Is that the way it's done?
3) Assuming I just wanted to create a JSON file for my 1.5.x core, what paths are expected in the ZIP? Do I assume that the IDE will extract the zip into a subdirectory of C:\Program Files (x86)\Arduino\portable\packages\arduino\hardware\sam\variants and the IDE will find the core there?
4) Assuming that I want to first fork the Arduino code from Github, to be as current as possible with the latest, will my 1.6.x based core be presented by the board manager as a siloed copy, or can my core simply have only the changed files as overrides to Arduino managed code?
Thanks