If you wanted to know, you would know. It's not exactly a secret. Here you go though:

https://www.eff.org/deeplinks/2020/03/google-says-it-doesnt-...

> Real-time bidding is the process by which publishers auction off ad space in their apps or on their websites. In doing so, they share sensitive user data—including geolocation, device IDs, identifying cookies, and browsing history—with dozens or hundreds of different adtech companies.