Linux referred to the usually free, unix-like Operating systems based on the Linux kernel and is GNU GPL based software. The large, commercial distribution was from 1992 through the licensing of the Linux kernel under the free GNU GPL. In 1983, Richard Stallman launched the GNU project with the goal to develop an Unix-like, POSIX -compatible operating system.
What is a file in Linux? What is file system in Linux? Where are all the configuration files? Where do I keep my downloaded applications? Is there really a filesystem standard structure in Linux? Well, the above image explains Linux file system hierarchy in a very simple and non-complex way. It's very useful when you're looking for a configuration file or a binary file. I've added some explanation and examples below, but that's TLDR.