How do we create tabs on pages?

There are some separate tabs for the same subjects on the Tv tropes pages, but sometimes different ones go on the same page (for example, manga and anime have different bars for the same page). How is it chosen which topic bar goes and which one stays (like some manga with popular anime adaptations just say manga)? Can you remove tabs and add new ones? I searched history and edit pages, but I couldn't find any indication of how they're done.

The "tabs" in question are Namespaces. The existence of an article with the same title in the relevant namespace causes the header tabs to be displayed.

In the case of those that go to the same page, those are what we call redirects, where we set up a page to directly link you to another page, which may or may not be in a different namespace. This is usually done for fairly close adaptations of works (like with manga and anime), or for cases where multiple works in a series are grouped on one page (so the pages for each of the individual works will point to the series page).