How do I make sense of navigation portion of the tutorial?

I’m trying to follow this: https://docs.nextcloud.com/server/13/developer_manual/app/tutorial.html#creating-a-navigation

But I can’t make much sense of the “Create a navigation” section. Where is part.navigation.php being pulled into the app? Unfortunately the complete app the tutorial links to on Github is considerably different now.

I found the equivalent file in the new example is: templates/fragments/navigation.php.