1.4 What are src/config.h, site-lisp/default.el, etc.?

These are files that come with Emacs. The Emacs distribution is divided into subdirectories; e.g., etc, lisp, and src. Some of these (e.g., etc and lisp) are present both in an installed Emacs and in the sources, but some (e.g., src) are only found in the sources.

If you use Emacs, but don’t know where it is kept on your system, start Emacs, then type C-h v data-directory RET. The directory name displayed by this will be the full pathname of the installed etc directory. (This full path is recorded in the Emacs variable data-directory, and C-h v displays the value and the documentation of a variable.)

The location of your Info directory (i.e., where Info documentation is stored) is kept in the variable Info-directory-list. Use C-h v Info-directory-list RET to see the value of this variable, which will be a list of directory names (after Info has been started). The last directory in that list is probably where most Info files are stored. By default, Emacs Info documentation is placed in /usr/local/share/info.

For information on some of the files in the etc directory, see What informational files are available for Emacs?.