You can find the template assignment configuration for all extra pages within the DESIGN > PAGE ATTACHMENT admin tool as seen here:

While extra page have the page template configuration they also have an additional "page body" component to them where you would set the "page body content" that would be placed within the page template assigned to that extra page. Both the page and body template for an extra page are set within the same admin tool page. You can see the page template configuration at the top of the page like so:

The "body template" would be placed within the page template where the the {body_html} tag is. You will see where to set that body template in a 4th section of that page that you see within the following screenshot:

Within that dropdown you will select the body template to use. Note that all default extra page body templates are by default within the /main_page/extra_pages directory of your custom template set. If you decide to make more we suggest you keep them there to save confusion.