At run time, master pages are handled in the following sequence:
1.Users request a page by typing the URL of the content page.
2.When the page is fetched, the @ Page directive is read.
3.The master page with the updated content is merged into the control tree of the content page.
4.The content of individual Content controls is merged into the corresponding ContentPlaceHolder control in the master page.
5.The resulting merged page is rendered to the browser.
Read more -
MSDN-How Master Page Work[
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MSDN-Nested ASP.NET Master Pages[
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