I think the more intuitive model (to me) is instead of thinking of it as a lightweight virtual machine, or a neatly packaged up OS, is to instead think of it as a process shipped with an environment. That environment includes things like files and other executables (like apt), but in of itself doesn’t constitute an OS. It doesn’t have its own filesystems, drivers, or anything like that. By default it doesn’t run an init system like systemd either, nor does it run any other applications other than the process you execute in the environment.