SITUATION EXPLAINED: Apple's September 9 event is John Ternus's first as CEO. • Tim Cook steps down September 1 after 15 years, becoming executive chairman. He's 65, Ternus is 51 • Ternus joined Apple in 2001 on the product design team working on the Apple Cinema Display, and started his career designing VR headsets at Virtual Research Systems in the late 90s • He ran hardware engineering for AirPods, Mac, and iPad before taking iPhone hardware, and was the youngest member of Apple's executive…