The document root is the home directory of your domain. The homepage is loaded from here as soon as someone accesses your website.
The document roots for your websites are found in the www folder of the Home directory.
For applications running on your server and for access via SSH or SFTP, the document roots are found in the directory x>.
A separate folder (y) is created for each website you set up in the Control Panel.
The name of this folder is changed by default to match the selected website. You can, of course, change these folder names.
The structure is the same for sub-domains.