The users and developers.
Since Linux is a collaborative development environment, everyone that has contributed to the project has usually written some of the documentation.
Members of the Linux Documentation Project. Individual package maintainers write manual pages for their software.
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By configuring the bootloader to use Windows as the default menu option instead of Linux. Refer to documentation on how to do this.
Many distributions have documentation or builtin graphical installation utilities.
The 'man' or the 'info' command have documentation about utilities and commands in the system.
If by "Linux software" you mean the Linux kernel, it is made up of code submitted by thousands of contributors from all around the world. Linus Torvalds (creater of Linux) is the one who has the final say on what makes it into the final operating system. If you meant applications that run on Linux, there are hundreds of thousands of organizations that write software for Linux. A lot of it can be found in the Ubuntu Software Center
Man (or manual) pages
You can find free eBooks on Linux through various online platforms such as Project Gutenberg, which offers a selection of public domain books, and websites like FreeTechBooks, which specifically lists free computer science and programming books. Additionally, the official documentation and guides from the Linux Documentation Project (tldp.org) provide comprehensive resources. GitHub repositories and online forums like Reddit also feature links to free Linux eBooks shared by the community.
Var directory is a sub directory of the root directory, The system writes data during process of its operation.