> Before header bidding, publishers sold ad space through a “waterfall” method, offering the space to one ad exchange at a time, typically prioritizing whichever had previously offered the highest prices. But Google made it so that its AdX got “first look” access through DFP by calling it to submit a real-time bid before other exchanges got the chance to take part in an auction. That meant AdX could buy up any inventory it wanted as long as it met the publisher’s floor price, then pass the less desirable space to other exchanges, according to the DOJ.

[...]

> But Google moved quickly to reestablish AdX’s power. It created a competitor to header bidding called “Open Bidding,” which let Google take an extra cut of revenue. And under the adoption of header bidding, Google’s AdX ultimately got a “last look” advantage when publishers chose to feed the winning header bid into their publisher ad server — which most often was Google’s DFP. That’s because AdX’s advertiser buyers would then have the option to bid as little as a penny more than the winning header bid to secure the most attractive ad space.[0]

Google's header bidding-related shenanigans were a big part of the antitrust case against them, and they were found to be "monopolizing open-web digital advertising markets"[1], so I wouldn't say that it is mostly not controlled by Google.

[0] https://www.theverge.com/2024/9/24/24253293/google-ad-tech-a... [1] https://www.justice.gov/opa/pr/department-justice-prevails-l...