When an aspx requested, IIS checks the file extension in its metabase.
To view the metabase Start> Run > inetmgr
In the Internet Information Services window right click in 'Web sites' > Properties
Select 'Home Directory' tab and click the Configuration button
Now you can see a table with 3 columns.
aspx is mapped to aspnet_isapi.dll
When IIS find the extension aspx, then it loads the aspnet_isapi.dll and pass all the request details to it.
aspnet_isapi.dll then loads the CLR and pass all the request details that received.
CLR then create HttpRuntime engine.
The begining of the runtime engine is HttpRunTime class. It creates the HttpContext object and stores all the request details.
From there HttpContext object passes through a pipeline which consists of HttpApplication object, HttpModule objects and finally HttpHandler object.
HttpHandler object creates the html page.