The containers are literally the "bolting on". You need to give the illusion of the software is running under a full OS but you can actually mount the system directories as read-only.
and you still need to mount volumes and add all sorts of holes in the sandbox for applications to work correctly and/or be useful
try to run gimp inside a container for example, you’ll have to give access to your ~/Pictures or whatever for it to be useful
Compared to some photo editing applications on android/iOS which can work without having filesystem access by getting the file through the OS file picker
