As Google’s chief executive, Sundar Pichai, tells it, his company has always been on the side of consumers. It has paid billions to other industry giants, like Apple and Samsung, he said, to make sure Google’s internet search engine worked as well as it should on those companies’ devices. Testifying on Monday in Google’s landmark antitrust trial, Mr. Pichai directly contradicted the Justice Department’s claims that his company’s huge payments to companies like Apple to be the default internet search option on their popular devices represented its unchecked monopoly power. Mr. Pichai said he had been worried that Apple, in particular, would make it more difficult to use Google’s search on its devices, and he believed Google had to pay to make sure that wouldn’t happen. “Given that Apple designs the experience, it wasn’t clear how they would change the experience if the financial incentive wasn’t there,” Mr. Pichai said while testifying for more than three hours.