John Ternus became Apple's chief executive on Tuesday, succeeding Tim Cook, the company's leader for the last 15 years. The long-anticipated handoff, Mr. Cook has said, will be "perfectly smooth." But Mr. Ternus will have to guide Apple through an unusual amount of leadership churn, stem an exodus of talent to companies like Open AI, and distinguish himself from Mr. Cook, who will remain as executive chair.

Mr. Ternus must display the financial discipline of the Cook era while showing Apple can still create products that capture consumers' imagination. At its annual hardware event this month, Apple is expected to introduce a foldable iPhone — its most significant change in 19 years to its most important product. The company is also taking another stab at weaving artificial intelligence through its products.

Over the past four months, Mr. Ternus, 51, has started installing his own executives. Apple hired Nate Gatten from American Airlines to lead government affairs, replacing Kate Adams. Laura Legros, a former hardware engineering vice president, has rejoined the company. More than 400 former Apple employees now work at Open AI, according to an intellectual property lawsuit Apple filed against Open AI in July.