The document root is the folder where you keep website files for a domain name. Since cPanel allows for multiple domain names (Addon domains), then you need to have a unique folder for each domain.
Your primary domain is rooted in the public_html folder.
Your addon and subdomain names will be rooted to their own folders inside public_html.
After you create an Addon domain (or subdomain), you can find the Document Root inside cPanel.
Tip: You can click the Document Root name in cPanel to go to the correct folder in the File Manager.