The Arduino sketch is uploaded as a process running in the OS. The board is compatible with the Arduino IDE and it's possible to write a sketch like in the regular Arduino boards, but that is not the only way to use the Galileo board. Regarding the second question, the Arduino part doesn't do anything with the OS, the sketches uploaded run as services in the OS. The latest sketch uploaded is stored in the "sketch" directory.
Thank you very much for all this explanation . BIG THANKS !