How is the Due / SAM core arranged in the Arduino IDE ?

Hardware Arduino Due
11

I know nothing about Ubuntu but, but in an IDE window, select File>Preferences, click in the URL at the bottom :

On my PC (and I guess this is more or less equivalent for you) this is C:\Users\xxx\AppData\Local\Arduino 15\preferences.txt

Then follow the path:
packages\arduino\hardware\sam....