When the Shop is loaded, one of the first things is to load the file virtuemart_parser.php. It makes core interactions like the joomla.php file (mainframe class) and after that looks for a parameter called page (can be passed by GET or POST).

The value of the page parameter is expected to contain both - pagename and the core module name:

shop.browse ⇒ shop is the name of the shop core module and browse is the name of the subpage.

By calling the URI index.php?com_virtuemart&page=shop.browse of your Joomla! site, VM will include the file

/administrator/components/com_virtuemart/html/shop.browse.php.