Hello,
There are no "variables". X-Cart 5 works differently. The page is made of a number of "widgets". Every widget is a PHP class that handles all the logic, and a template file that uses methods provided by the class. So, there is no single place where "variables" are defined. Instead, there are multiple classes that define methods supplying data to templates.
You should enable the webmaster mode and determine what existing widgets render the data you need. Then check the source code of these widgets and implement a similar logic in your custom widgets.
For developer's documentation please refer to the portal at
http://devs.x-cart.com/