Think of the Table of Contents tab as your course outline. Clicking on any item in the outline will access that content. While some content can be created under the Table of Contents, it may also be built elsewhere (in the various tools) and connected via Add Existing Activities.
Content can be built in more than one place in D2L. D2L uses tools such as the Quizzes tool, Discussions tool, or Assignments tool to create content for your course. It's best practice to build items in the related tool, e.g. build your quiz in the Quizzes tool. These tools are found under the Navbar menus.
Some items, such as pages, can be created directly in the Table of Contents. However, content may also be uploaded to D2L, and in that case must be linked via Upload/Create > Add from Manage Files or Upload/Create > Upload Files dropdown. Items created by tools or uploaded must be linked via the Add Existing Activities dropdown menu. For example, after creating your quiz in the Quizzes tool, you would need to link the quiz to the Content outline via Add Existing Activities. Creating an item in a tool does not automatically add it to the Table of Contents outline for you. Deleting an item from the Table of Contents does not remove it from the tool.