The people that run websites decide to show adds by adding a piece of software provided by Google. The website receives money from advertisers and Google gets a cut. They have a similar thing for Ads for apps. What Google does is collect data and uses advanced algorithm that decides which ads go where and who sees them to maximise revenue and value to advertisers.

Facebook works differently because it collects data about users (using shady tactics) and shows ads on its own platform from advertisers that want ads solely on the social media.