Where is Apache'e Default Document Root Directory?

You can find Apache's default docroot by running this command: 

grep -i dir_docroot /etc/cpanel/ea4/paths.conf
dir_docroot=/var/www/html

Usually, these files are found inside the directory: 

.
├── 400.shtml
├── 401.shtml
├── 403.shtml
├── 404.shtml
├── 413.shtml
├── 500.shtml
├── cp_errordocument.shtml
└── index.html

Please bear in mind that this directory contains the default document root for the server. It contains only the default pages that users can see and it is a reserved directory, so it should not be used for setting up other content. The proper location for user-specific content associated with virtual hosts can be found by running this command: 

grep -i homedir /etc/wwwacct.conf
HOMEDIR /home

When you create a new user/account in cPanel a new subdirectory is automatically created inside this directory where all of the user's content resides. Consequently,  the web root directory for a given user would be found here:

/home/$USER/public_html

All the web content for the account will reside in this location. This directory is also accessible from the cPanel > Files > File Manager > public_html