This is a question that can confuse many people, after all, Linus Torvalds stated in his initial email “I am making an operating system”. Linux, contrary to what many people think, is not an operating system, but a kernel, the core of an operating system and therefore the most important part. We can think of a car as the system and the engine would be its kernel.
Linus developed a Kernel, which is already something very difficult to do and keep up to date even with the help of a whole community. While the FSF (Free Software Foundation) started with the applications and then tried to develop the Hurd kernel in their GNU project, Linus started with the kernel and never got around to developing the applications, while the FSF hasn’t finished the Hurd yet.
As he didn’t have the applications, Linus used extensively the FSF applications that worked on the Unix system, so if they also worked on Linux it would be a good sign that the kernel would be fine-tuned since Linux was similar to Unix.
That’s why the name Linux only refers to the kernel created by Linus Torvalds when we use the name Linux to refer to a system we are referring to a GNU/Linux operating system, that’s the correct form, but we ended up using only Linux for the sake of convenience and also because of a bit of unfamiliarity.
