Analizing a code

Joshua Bloch said in an interview that the best way to know and understand code written by others is to read it , mentally or through a debugger . Well, my question is this: is it possible , through a debugger , analyze the code of a web application? I'd like to understand the Magento code, see the classes, the methods that are called when I click , I open a page , fill in the form, place items in the cart , in short, is it possible and how to do it? (sorry my bad english)