They get that information from tracking you across the wide internet (because you see the banners which also track you) and they sell targeted advertisement to

They get that information from tracking you across the wide internet (because you see the banners which also track you) and they sell targeted advertisement to specific audiences using real time bidding systems where you can bid for advertisement space for a specific audience ('interested in cars', 'interested in electronics', 'probably male', 'age 30 to 40') etc. That profile data is gathered/derived from your browsing behavior.