72 or 144 hours transit free visa

I am a German citizen. What are my options when I travel to Cambodia from London via Beijing or Shanghai?
May I avail 72 hours in Beijing or 144 hours twov in Shanghai and have a visit to Beijing wall and other places in Beijing or not?
If I land at Shanghai Airport then May I travel by fast train to Beijing and back to Shanghai after visiting China Wall and some other places in Beijing and take my flight to Cambodia?
Waiting for your kind reply.
Best regards