The Apple executive was testifying in a federal court in Washington as part of the Justice Department's lawsuit against Alphabet. Last year, a U.S. District Court judge ruled that Google had illegally dominated advertising technology markets, and now the judge is looking to determine what penalties or actions should be taken against the search company.
One major focus on the case is Google's practice of paying platform providers such as Apple to become the default search engine on their platform.
The lawsuit and any potential remedies threaten Google's lucrative advertising business, and Alphabet shares sank more than 7% on Wednesday.