SAN FRANCISCO — The man behind the iconic designs of the iPhone, iMac and iPad is leaving Apple. The company said Thursday that Chief Design Officer Jony Ive is leaving after more than two decades at Apple to start his own design firm. But he’s not completely severing ties. Apple said it will be one of Ive’s clients at his new firm. The Cupertino, California, company did not give an exact date for his departure. Ive has been a fixture on Apple’s design team since the early 1990s and is known for shaping Apple’s signature rounded, stylish designs. He won’t be immediately replaced. Two of his deputies will report directly to the company’s chief operating officer, Jeff Williams.