I would highly recommend reading the decision. The two markets defined were publisher ad servers (i.e. Google's DoubleClick for Publishers) and ad exchanges (i.e. Google's AdX). Google behaved anticompetitively in both of these markets.

DoubleClick for Publishers placed restrictions on how publishers could work with non-Google ad exchanges and tilted things in favor of Google's exchange. Third-party exchanges weren't given information about the specific impression before bidding and an advertiser bid submitted on AdX could win even if there was a higher bid submitted on a third-party exchange. DFP also banned publishers from setting higher price floors for AdX than for third-party exchanges, but allowed setting higher price floors for third-party exchanges than for AdX.

In response to Google's unfair ad auctions, publishers eventually started using "header bidding", a technique for getting real-time competitive bidding from multiple ad exchanges. However, Google still controlled the largest ad-buying platform, DV360. DV360 was the top buyer on every third-party ad exchange in addition to AdX. Google modified DV360 to automatically lower bids submitted to third-party ad exchanges such that they were always below an advertiser's maximum budget. When the publisher ad server received the bid, it would use it as the floor price to solicit more bids on AdX. DV360 would then bid the publisher's maximum budget via AdX, taking the win away from the third-party exchange.

The decision also goes into why ad networks are not a substitute for the combination of publisher ad servers and ad exchanges: "Although ad networks are another tool for connecting advertisers to publishers, the sophisticated publishers who receive the majority of open-web display advertising revenue do not view ad networks as substitutes for ad exchanges because ad networks offer very limited control and are unable to place bids from disparate demand sources in competition with each other." The government never claims that Google has monopoly power in the publisher-facing ad network market (i.e. AdSense).