How URL Mapping Works I am new to Craft and I am trying to figure out how routing works. When I click on an entries nav item, it makes a request to:

.com/index.php?p=admin/actions/elementIndex/getElements


The url of the item is .com/entries/affiliates and the call is ajax. 

There is a section called affiliates, but the 'template' field is Null. I can't seem to figure out where the response is coming from.
There is an overview of how routing works in Craft here: https://craftcms.com/docs/routing and that guide works for front-end routing as well as Craft's own control panel routing when deciding which (if any) templates to render.

The call to p=admin/actions/elementIndex/getElements is a control panel AJAX action request (#3 is the routing list) that's going to the craft/app/controllers/ElementIndexController.php's actionGetElements method and returning data.