Two basic things:

IA is not nav. Expand...

On mobile, especially, nav is not nav. The menu or tab bar is not The Navigation, but anything that gets you there is. Often, the list of features on a dashboard, or the list of posts on twitter. That's nav. Expand...

What follows are a couple specific nav structures, but many others are in other sections. They are there, like tabs, because they do not work universally well like we convinced ourselves tabs and sidebars did for desktop web. They have specific sub-uses and are grouped like that.