Part time work in Germany can be: work for the minimum wage (Minijob), in the form of a small individual business or work on honorar. You can earn extra money while having a main paid job at the firma, but also a main unpaid job in the family or being a student.
For example, many come up with the idea to somehow monetize their wonderful photos. The entrance to such an activity is very simple – register in the stock and sell. Or work in a Verein, as a tutor, filming events on weekends, babysitting…