Arturo Bejar, who worked at Meta from 2009 to 2015 and as a contractor from 2019 to 2021, testified on Tuesday in a federal trial in Oakland, Calif., scheduled to last six weeks. A coalition of states alleges that Meta designed platforms to addict young users, misled the public on safety, and illegally collected data from children under 13. Meta denies the allegations and argues that Bejar’s opinions exceed the scope of his work.