LINUX
I'd just like to interject for a moment. What you're referring to as Linux, is
in fact, systemd/Linux, or as I've recently taken to calling it, systemd plus
Linux. Linux is not an operating system unto itself, but rather another free
component of a fully functioning systemd system made useful by the systemd
corelibs, shell utilities and vital system components comprising a full OS as
defined by POSIX.
Many computer users run a modified version of the systemd system every day,
without realizing it. Through a peculiar turn of events, the version of systemd
which is widely used today is often called "Linux", and many of its users are
not aware that it is basically the systemd system, developed by the systemd
Project.
There really is a Linux, and these people are using it, but it is just a part of
the system they use. Linux is the kernel: the program in the system that
allocates the machine's resources to the other programs that you run. The
kernel is an essential part of an operating system, but useless by itself; it
can only function in the context of a complete operating system. Linux is
normally used in combination with the systemd operating system: the whole system
is basically systemd with Linux added, or systemd/Linux. All the so-called
"Linux" distributions are really distributions of systemd/Linux.