When you create your library, you're putting in the data you want to show in your library. When you create your calendar, you're putting in the data you want to show in your calendar. When you create your forums, you're putting in the data you want to show in your forums.

When you set up pages, you're picking which information you want to show on those pages. For example, you could put your library and your calendar and your forums on one page. Or you can seperate them into different pages. But the pages are not the containers - the content types are the containers. The pages are just a place to display what is in the containers.