I am part of verdi and they helped me more than once to get money the employer did not pay. They also help in other tricky situations. I can only recommend becoming a union member. You have a work related legal insurance, where you actually never have to pay anything extra (compared to private legal insurance).

In addition, no matter how much you need them, they can't throw you out. I have a couple of firends who are not in the union and have private insurances for years. Corona came by, they asked to sue their employers for a compensation after being fired and the insurance is like " ok, but now since you need help you are a high risk client. We will finish this case and then we won't work with you anymore, bye."

Unions are a great thing and the reason why we have so many rights in Germany that other countries don't have. Especially young people don't see that, because they have the mentality of "If you don't like it, go somewhere else. Why are you making such a fuss?"

Sure everyone can leave to a different job. But this doesn't change the behavior of the company. The contrary is the case. They teach companies that it is enough to treat your employee like a piece of shit for a couple of weeks and then they will just leave without any consequenses.

You can't choose the union you want to join (thanks to Merkel, who destroyed most of the unions), there are several unions for different job groups.