In symfony, pages are composed of an action (a method in the
actions/actions.class.php file prefixed with
execute) and a template (a file in the
templates/ directory, usually ending with
Success.php). They are grouped in modules, according to their function in the application. Writing templates is facilitated by helpers, which are functions provided by symfony that return HTML code. And you need to think of the URL as a part of the response, which can be formatted as needed, so you should refrain from using any direct reference to the URL in action naming or request parameter retrieval.
Once you know these basic principles, you can already write a whole web application with symfony. But it would take you way too long, since almost every task you will have to achieve during the course of the application development is facilitated one way or another by some symfony feature ... which is why the book doesn't stop now.