Depend on what you are asking. Do you wont to know the logical flow or the technical. Logical is defined by the JSR standard so probably you like to know what's the implementation.
So Browser HTTP request -> app server -> Filters > MainServlet (Liferay entry point) > Service Pre actions > Struts Framework > depending on the life-cycle phase an action may be executed > rendering in JSP > all portlets on the page are called and provide they're HTML fragment > theme and fragments are combined > final response is created and send to browser
