the [contact] is a shortcode, you can find the function (function contact) in the file “shortcodes.php” on line 914, the file is located in the “7league” folder.
If you want to add more information to the contact form, it’s better you use a contact form plugin, because if you change our contact form shortcode, you have to change also the file for the ajax request. That’s why I think it’s easier to use a plugin.
The contact form was made to have a small build-in contact form if you don’t want to use a plugin for it. But it’s only a simple contact form…