I'm flying from Chicago to Shanghai on April 6th, with a 3 hour layover in Beijing.

Where do I apply for the 72 hour free transit? In Beijing or Shanghai? ( I don't plan on leaving the airport in beijing). Does the 72 hour visa start as soon as you arrive in the country or when you leave the airport? I land in Shanghai on the 7th at 10:55 am and leave on the 10th at 6:50 am.