Osaka to Xian to Seoul with brief layovers question

I have a US passport. Flight from Osaka to Xian has 2-hour layover at Shanghai PVG before flight continues to Xian. If I do not leave the airport in Shanghai, can I still use 72-hour no visa policy for my visit to Xian? Something similar happens on departing flight (Xian-Shanghai-Seoul). Once again I would not leave the Shanghai airport. Please advise. Thank you.