The source – support.google.com
To set up a link, in the AdSense console, go to the Optimization tab, then click Labs. There will be "Include AdSense links".
How it works
Google finds text in the main content of the page, for example, it is a product that can be purchased. Then inserts the appropriate link in the memory. When you click on it, users see product ads.
Google promises to limit the display of product links to 10% of traffic.