If its a link very often there is a part behind the normal link to a site that tells the website where you came from. It is usually something like reddit(dot)com/r/eli5/thispost?asdhjahjsdjkasdjghk the part behind the question mark then could tell me you got to this post through an add on facebook or google or what ever (there are other ways to do this but this is a very common one). After that websites save cookies to identify you and can match where you came from and what you bought.