if i fly from tokyo to singapore ,, and the planes stops in shanghai to pick up more passengers

No, u cannot. To use the 72h visa-free policy, passengers should have only one stopover in a domestic city. Anyway, if your total transit duration (in Shanghai and BJ) is within 24h, you can apply for a stay permit in Shanghai, with which you will be allowed to go to Shanghai downtown and BJ downtown for a visit!