Each Channel has its own Frontend, which is responsible for abstracting the communication for the rest of the application. The diagram shows three different Frontends. There could be many others, though.
For simplicity’s sake, the diagram shows only the EntryPoint script and the I/O classes. The Frontends encapsulate everything that is Channel dependent, so they are much more complex.
This is mostly the HTML View from Joomla! < 4.0. It retrieves the Request data and provides it in a Channel independent form. The HTTP Frontend handles all the HTML rendering, module gathering, and other things we use to see in the CMS as a web application. Different Renderers can be used to output HTML with jQuery, HTML with MooTools, PDF, CSV, … you name it.
This is mostly the JSON View from Joomla! < 4.0. It retrieves the Request data and provides it in a Channel independent form. The REST Frontend formats the result accordingly.
The command line options and arguments are provided in a Channel independent form in an Input object. It echoes or stores the resulting data, depending on default configuration or command line switches.
On the output side, the Content is transformed to the output format by a Renderer using the Visitor pattern.