The Rendering of the page is handled by a collection of Renderers. The rendering begins with the PodContainerRenderer which recevies the document from the loading process and generates the PodContainer widget. It then delegates the rendering of Pods to a Pod renderer which in turn delegates to other renderers using markers in the data it receives. Each renderer returns its own content which it either generates itself or generates with the help of other renderers. This pattern of delegation is repeated unit all content is rendered.
The Curam Widget Development Guide discusses in detail the rendering framework and how renderers interact.