Info files are created from Texinfo source files. You can use the same source file to make a printed manual or produce other formats, such as HTML and DocBook.
The makeinfo command converts a Texinfo file into an Info file;
texinfo-format-region and texinfo-format-buffer are GNU
Emacs functions that do the same.
See Overview of Texinfo in Texinfo: The GNU Documentation Format, for how to write a Texinfo file.
See Creating an Info File in Texinfo: The GNU Documentation Format, for how to create an Info file from a Texinfo file.
See Installing an Info File in Texinfo: The GNU Documentation Format, for how to install an Info file after you have created one.