Every UNIX OS has three files open all the time for its processes. Remember that UNIX considers everything, even a printer or your mouse, a file.
UNIX uses file descriptors, which are positive integer values, as an internal representation for accessing all of its open files, which is much prettier than using long paths.
So, by default, all UNIX systems support three special and standard filenames: /dev/stdin, /dev/stdout, and /dev/stderr, which can also be accessed using file descriptors 0, 1, and 2, respectively. These three file descriptors are also called standard input, standard output, and standard error, respectively. Additionally, file descriptor 0 can be accessed as /dev/fd/0 on a macOS machine and as both /dev/fd/0 and /dev/pts/0 on a Debian Linux machine.
Go uses os.Stdin for accessing standard input, os.Stdout for accessing standard output, and os.Stderr for accessing standard error. Although you can still use /dev/stdin, /dev/stdout, and /dev/stderr or the related file descriptor values for accessing the same devices, it is better, safer, and more portable to stick with os.Stdin, os.Stdout, and os.Stderr offered by Go.