Any cell on a page can be converted into a menu widget. Often this would be in your header, but you could also add a secondary menu within a page content area, or perhaps in your footer.
The menu widget is built automatically from the structure of your pages in Pages Manager. You can create and apply menu styles to achieve the layout, styling, and behaviour of the menu widget that you require.
You can aslo define what the root page of your menu widget should be. It does not have to display the entire site structure. For example if you have a sub menu within a page, you might set the root page to be the parent, and then the sub menu will only show it's siblings.