From a Wall Street Journal online article:
In interviews, dozens of publishing and advertising executives said Google is doing just that with an array of interwoven products. Google operates the leading selling and buying tools, and the biggest marketplace where online ad deals happen.
Overall, Google made $116 billion in advertising revenue last year, a 22% rise from the previous year and 85% of the company’s total revenue. Most of that ad revenue came from Google’s own properties, but the company’s vast role in brokering online ad sales off its own platforms gives it an added level of dominance.
Alphabet Inc. ’s Google is under fire for its dominance in digital advertising, in part because of issues like this. The U.S. Justice Department and state attorneys general are investigating whether Google is abusing its power, including as the dominant broker of digital ad sales across the web. Most of the nearly 130 questions the states asked in a September subpoena were about the inner workings of Google’s ad products and how they interact.