Google owns 90%+ of search, Android, and the Ad side. The three together allow it to crush competitors from both ends all while taking a 35% cut. The most lucrative part of the ad business is paid downloads. how many of those searches start in Google with e.g. "Download Spotify". Google makes money selling that to Spotify and they can do it at a much higher rate because no other ad seller has access to search inventory for "download Spotify" or similar searches.