how it starts..whether from web.xml or config.xml.?
Configuration files do not start anything, they are just text files that are read in by pieces of code -already running- to configure themselves and possibly other components as well.
The fact that you have to ask these questions and the fact you are apparently completely unwilling to read documentation is alarming. Do you at least know how a basic servlet/JSP environment works? How a servlet engine like Tomcat is used?