Under cross-examination, Cook sparred with Epic lawyer Gary Bornstein about the profitability of the App Store.
Cook disputed Epic's contention that its profit margin on apps was some 80 percent, but financial details were not disclosed in court due to confidentiality concerns.
The Apple executive said the proprietary payments system challenged by Epic was about convenience for consumers, more than about profits.
"We always put the user at the centre of everything we do," Cook said. "It has nothing to do with money."
During his testimony, Cook defended Apple's policy of barring apps directing consumers to other platforms to purchase subscriptions or credits for games and other services.