Two stops with long layovers, do I need a visa?

I am coming from Thailand on the way back to the United States. I have one stop on kunming with a 21 hr. Layover and one stop in Shanghai for 9 hours. Do I need to get an entry permit? Or does the 72 hr. Free travel apply? I would like to get out of the airport during the super long layover.

United States passport holder