Does AdSense send a page's HTML to Google for analysis? If not, how?

Question

I'm thinking about making an ad network as an experiment and for learning a new server language.

I can't understand how Google AdSense knows what HTML is on a page. The request to googlesyndication.com appears to send them the current page's URL, but no HTML. How, then, do they know what is on the page?! Pages will look different for different users, even on the same URL. Some URLs will be behind passwords, yet AdSense still shows relevant adverts.

I don't understand what they are doing. Can you tell me how they do it?

Answer 1

It checks whatever Google has cached in their history for that page to determine the ads it displays.

Answer 2

Content behind usernames and passwords can be crawled by Google if you provide them with access to the content. This can be done in your Google Adsense account -> AdSense Setup -> Site Authentication.

Use this section to allow the AdSense crawler to access pages that are behind a login. Our crawler will access these pages only to determine content for ad targeting purposes and will fully comply with Google's privacy policy