That's already public information at this point, following Google's 2020 antitrust case.
[1]https://www.forbes.com/sites/enriquedans/2021/01/19/jedi-blu... [2]https://www.wsj.com/articles/googles-secret-project-bernanke...
I assume it ultimately goes back to the search engine, somehow.
EDIT: I found an interesting article
https://law.stanford.edu/publications/why-google-dominates-a...
Abstract: Approximately 86% of online display advertising space in the U.S. is boughtand sold in real-time on electronic trading venues, which the industry calls “advertising exchanges.” With intermediaries that route buy and sell orders, the structure of the ad market is similar to the structure of electronically traded financial markets. In advertising, a single company, Alphabet (“Google”), simultaneously operates the leading trading venue, as well as the leading intermediaries that buyers and sellers go through to trade. At the same time, Google itself is one of the largest sellers of ad space globally. This Article explains how Google dominates advertising markets by engaging in conduct that lawmakers prohibit in other electronic trading markets: Google’s exchange shares superior trading information and speed with the Google-owned intermediaries, Google steers buy and sell orders to its exchange and websites (Search & YouTube), and Google abuses its access to inside information. In the market for electronically traded equities, we require exchanges to provide traders with fair access to data and speed, we identify and manage intermediary conflicts of interest, and we requiretrading disclosures to help police the market. Because ads now trade on electronic trading venues too, should we borrow these three competition principles to protect the integrity of advertising?
Mostly because they actually are the best for most advertisers, both in terms of raw offering (publisher portfolio i.e. "reach" and the tools to understand how your ad campaign is doing and tune it) and the third-party ecosystem of tools that have grown up to manage an ad account. Possibly not the best for publishers (see horror stories of failing to cash out in other threads on this story), but when most of the ads you want to run are in one network, you'll go there. Plus, for all its flaws and the failures that occur from it being a high-value target, for a publisher Google still somehow tends to be the automated ad-matching house least likely to match your readers to scamware that will make them unlikely to come back to your site.
Turns out the problem of automatically matching ads to publishers to human eyeballs is legit-hard and only a few companies have the technical competency to do it well at scale.