1) User requests the asp.net  page (page having the extension .aspx)  by typing the URL for the page in the browser.

2) Server receives the request for .aspx page and executes the code in the code behind of the page.The output of the page is HTML which is sent back to the browser. 

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Also please understand that the browser understands only HTML so whatever you request the output has to be the HTML.

Now what happens when the page executes :

ASP.NET page has a lifecycle which it undergoes .The major events that are fired when the page goes through the lifecycle are :

Page_Init

Page_Load

Page_PreRender

Page_Render

Page_Unload

You can write code in any of these event handlers(more event handlers are there) to change the generated HTML.