Many computer users run a modified version of the GNU system every day, without realizing it. Through a peculiar turn of events, the version of GNU which is widely used today is often called "Linux", and many of its users are not aware that it is basically the GNU system, developed by the GNU 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 GNU operating system: the whole system is basically GNU with Linux added, or GNU/Linux. All the so-called "Linux" distributions are really distributions of GNU/Linux.
It's too bad RMS got sidetracked with Hurd. But, the GNU system now runs with several kernels - the Linux kernel is just the most developed and best known one.
Many computer users run a modified version of the Darwin system every day, without realizing it. Through a peculiar turn of events, the version of Darwin which is widely used today is often called "macOS", and many of its users are not aware that it is basically the Darwin system, developed by Next Computer.
There really is a macOS, and these people are using it, but it is just a part of the system they use. XNU 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. Darwin is normally used in combination with the macOS operating system: the whole system is basically Darwin with macOS added, or Darwin/macOS. All the so-called "macOS" versions are really versions of Darwin/macOS.
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Apple's engineers still refer to the OS as Mac OS X. Ventura is technically 10.18, despite the 13 major number in their marketing.
No.