“I want to make it clear to everybody exactly what was happening on these platforms,” Mr. Sweeney said from a witness stand on Monday. “I want everybody to see and understand Google exercises de facto control over the availability of apps on Android.”

In 2021, a federal judge rejected most of Epic’s arguments in a similar case against Apple. This time, a nine-person jury will decide whether Google violated antitrust laws by exploiting smaller rivals in a trial that is expected to conclude next month.

The outcome could have wide implications. If Epic wins, Google could be forced to allow other companies to offer competing payment systems on the Play Store.

Since the trial started two weeks ago, Mr. Sweeney has been sitting in the front row of the courtroom almost every day. He was determined to go to trial alone: Google announced a settlement last month with the other plaintiff in this case, Match Group, a dating app company. In September, Google reached a settlement with dozens of state attorneys general who sued the company on similar grounds.

In his testimony, Mr. Sweeney insisted that his goal was to distribute the games to more users, rather than seek monetary damages, and that Google’s fee prevented Epic from expanding its business. In a cross-examination, Google’s lawyer, Jonathan Kravis, said Epic also paid gaming console companies, including Nintendo, Sony and Microsoft, a 30 percent commission fee and had made $12 billion from the consoles together.