I'm a US citizen. Traveling from US to Cambodia transiting through Shanghai.

1. Do I need a transit G type to go through Shanghai to Cambodia?

2. On the way back from Cambodia to US, I am going through Shanghai again.
Do I need a visa to stay 2 or 3 days to travel to Chongqing, China and then come back to either Shanghai or Bejing to fly back to US?

If I need it, what are the requirements, where to get it, and the cost please?

Thank you for answering questions.