Do you play Harry Potter or meet nerds just because people read about it in a book, and would you do it if your book was good too?

With your massive mistakes in the question, you should first think about your German skills and expand them before you want to write a good book. (Only by the edge. I had to read it 3x so that I understood what you want.)

You would have to write a very good book and have much more luck, that it is seen and spread, like Harry Potter. As a self-publer, this becomes difficult, because there are all costs with you. You have to write, rework, send to test readers, revise it, commission a editorate and correctorate, print a cover designer and, of course, also books. Because as a pure eBook you will hardly succeed.
You need to keep readings, create merchandise and sell at fairs with books, advertise, write bloggers, create pages, print reading samples, draw attention to you. It costs a lot of money and time.

If you convince a publisher, he will take most of the work off you. But you don’t get the 70% like Amazon.
As a rule, after deducting VAT, an author receives between six to ten percent of the book price as a fee. This would be only EUR 0.93 per sold copy for a 9.90 book.

So you have to be insanely lucky that your book is going to be well and bought. It’s even more difficult than a 6 in the lottery. I already know many writers who are all writers next to the profession, because they cannot live by writing alone. But they love it and this is the main thing, I think. Of course you can be lucky, but it’s not as easy as many people think. I was often smiled by people from my environment because I wrote books and only now after 20 years I could fulfill my dream, because self-publer did not come into question for me. That’s too much work. I have found a reputable publisher who wants to trade my books (without asking for money, as some DKZV (compression cost supplement).)

If your book is sold well, it may be that you or your publisher sells it abroad in several languages. Listening games are made and if all goes well, maybe the film industry comes and if that goes, everyone comes and wants to make money. Then the story is slaughtered. From bed sheets, pencils, clothes to video games and co.