Subdomains

My shared server account uses cPanel. Can someone explain to me how cPanel tells the server to send a browser that enters http://blog.domain.com to open index.htm in http://www.domain.com/blog? There is an entry in Subdomains that does the redirect, but where does cPanel write that information on the server drive?

Second question. When I type in http://webmail.domain.com, how does the server know to redirect to http://domain.com:2095?