I found that some people use a bike-sharing service, the one similar to mobike/ofo in China.
For example I found a bike below, which one person rode and left it behind, and then another person used it.
But is it possible for a traveler to use one of them? I don't have an official residence in Germany (non-EU citizen), don't have a bank account in Germany, and don't understand German language, for your information. I have a phone number and passport, and temporary address on a room booked on AirBnB.