The core operating system is a single read-only file (ROM, as in custom ROM on Android) and all the user files and customizations are on a different partition or such. Since the core system is fixed you can just swap it with a newer ROM when updating (and also go back to the old one if the update fails somehow.).
Can you explain what image based OS means?
The core operating system is a single read-only file (ROM, as in custom ROM on Android) and all the user files and customizations are on a different partition or such. Since the core system is fixed you can just swap it with a newer ROM when updating (and also go back to the old one if the update fails somehow.).