Google gains revenue via several ways. Google's primary source of revenue is from paid advertisements across all there services. By providing a large amount of services and for features free, Google gets a large amount of users using their services all the time and that can categorize them into specific audience segments. Ads go across search, maps etc and services like Google Adsense have millions of websites using it. That's about 95% of their revenue. The rest of the revenue gets split between paid services (Google Cloud, API's) and royalties on Google Play and Android hardware