How do I use dynamic content inside my content objects?

You shouldn't. It's that simple. Really. :)

If you need a special view that pulls data from other locations into the content area, you should put that logic in a template, and not in the content objects.

There are a lot of good reasons to not mix content and logic, and Plone enforces this. If what you are looking for is to combine elements of different content (say, a document with a content listing), there are products that let you do this without necessarily creating your own templates. mxmDynamicPage, CMFCollection and CMFContentPanels are some examples - but shop around, there is a lot of development in this area, and this FAQ entry might be slightly outdated.

Do not use CMFDynamicDocument or AtDynDocument or any kind of tool that lets you put logic inside content. It is bad practice, destroys maintainability - and works directly in opposition to what content management is about.