The DXA site navigation is documented here http://docs.sdl.com/LiveContent/content/en-US/SDL%20DXA-v6/GUID-4A757BE5-AED2-4AEE-B9A0-E2A1BEE30C49
There are two ways to implement navigation:
- Structure Group hierarchy (default)
- Taxonomy based (a Category and its Keyword hierarchy, which are published)
You set the navigation type while installing the Web application, which must have suitable Navigation Taxonomy in order for the second navigation type to work. In the absence of such a Taxonomy, the system falls back to Structure Group-based navigation.
When using the navigation based on Structure Groups (i.e. the default or fallback), all you have to do to include either a Structure Group or Page to the navigation, is ensure its title starts with a three digit number followed by a space (i.e 020 My Page). Once the Page is Published, you have to republish the _Navigation Page, as described here http://docs.sdl.com/LiveContent/content/en-US/SDL%20DXA-v6/GUID-33769114-BCF6-46C2-8FA6-24F1FD88E444
The Page metadata Schema "Page Navigation Metadata" is part of the Taxonomy-based navigation. Navigation is then based on the Category called Sitemap [Navigation] (part of the 400 Example Site Publication). In order for a Page to appear in the navigation, it would need to be associated with one or more Keywords from the Category (through the Page Navigation Metadata Schema), and after making changes (either in the heirarchy or adding removing Pages from the navigation) you will have to republish the Category.