Stepping into his new role as Apple Inc.’s chief executive officer on Sept. 1, John Ternus won’t have much time before the company begins its annual fall product rollouts. This year the lineup will include an unusually ambitious slate of devices he’s been teasing for months internally, including the first foldable iPhone and a smart display that can recognize who’s speaking and tailor content to them. Ternus spearheaded those devices in his previous job as Apple’s hardware chief, while also over...