Turkish & German passport holder, flying from South Korea to Istanbul

This is very simple. Since you have a German passport, you will use it because German citizens do not need a visa to enter China for up to 15 days. You would have also a right to travel via Shanghai with Turkish passport visa free, but since your layover is 8 hours and 30 minutes, you cannot know whether the airport authorities would allow you to exit or not. Traveling on German passport will allow you to exit the airport, definitely, visa free.

Shortly before landing in Shanghai, the crew will dispense arrival cards on board the plane. You will fill out one for tourist/sightseeing purpose and just head straight toward the immigration with that card and your German passport. That's it.