Inorder to change the quiz module. I would like to know where to start to understand the quiz module.
Want to know how the questions are rendered by using html and the database interactions.
Your question is very, very broad, and difficult to answer without more information. Are you a programmer? Are you a PHP programmer. What do you know so far, what code have you looked at?
Actually, http://docs.moodle.org/dev/Question_API is probably a better place to start reading.
Looking at the code itself is also highly recommended.