In Joomla! there is a default template, but you can assign other templates to specific "pages" that are defined by menu links.
To assign a template to a page, you must first make sure that there is a direct menu link to the page.
Note that you cannot assign the default template to individual pages.
The templating system uses the ItemID to determine which template to show. ItemIDs are created when you create a menu link. This is why only menu items are shown in the list of pages to which you can assign templates.