From source to pages
The left box is the content sources under the content root. The middle box is the build pipeline that reads them. The right box is the published pages, each one derived from a source rather than authored directly.
What the diagram is not
The diagram describes this hub’s authoring flow, not a released product’s architecture. The application itself stays externally hosted and is reached through an ordinary HTTPS link.
The middle box is described in advanced patterns; the overview says which surfaces the right box publishes.