Override to master pages

Question

I am working with a C#.asp.net 2010 web forms application that I need to modify. Since I am new to asp.net (i Have other C#.net expereince), i have the following questions:
1. In this application all the web pages at the start looking basically alike except for one where you can select 'menu' items from. The one web page where you can select mwenu items from, must being overriden with ajax and/or a content area? can you tell me to look for to determine how any area of the master page is being overwritten?
2. When I look at the source code behind a webforms designer (look at the HTML), I only see the code for content place holder #4. This is the actually processing for the web page. This does not show the html (code) for the master page. (THe master page is gray and the application will not let me click on the master page section.) Thus can you tell me: 1. What can I do to see the master page code? 2. Do I need to learn something about how the master page is 'wired in'? Can you explain this to me and/or point me to a url I can use as a reference?