E-commerce giant Amazon uses a concept called collaborative filtering to achieve this. Collaborative filtering is a process of comparing the users with similar shopping behavior in order to suggest products to other users with similar shopping behavior. The engine makes predictions of what might interest a user based on the preferences of other users, and all it has is DATA. The data it contains INCLUDE users browsing history, recently ordered products, wishlisted products, etc.

For example, a sales record shows that a certain number of people who buy a phone ALSO buy tempered glass ALONG with them. So if next time a user buys a smartphone, he will receive a recommendation to buy a tempered glass as WELL.