IMPORTANT UPDATE / NOTE:
All my apps that were previously removed (months ago) due to an alleged violation of Google’s “Repetitive Content” policy have been re-allowed and listed in the Play Store again.
I still don’t know the exact criteria with which the Repetitive Content algorithm works.
It seems to be top secret. 
Since I did not receive a precise answer from the Google Dev Support, I assume that even they do not know this algorithm. 
However, it seems with a high degree of certainty that no app is allowed to contain parts (even if these are not the main subject of the apps) that are already included in other apps and target the same countries.
In other words:
In some of my relaxation apps there was a screen on which nature sounds (background sounds) could be played (heard as a sample). These apps had different content (different relaxation methods) and were in different languages (English / German), but they targeted the same countries.
Solutions in my case:
- I removed this sounds screen from all other apps targeting the same countries or
- I published the apps that still contained this screen in different countries.
- As a precaution, I removed the free version of an app (PMR Lite) from the Play Store (i.e. withdrew the publication), for which there was also a paid version (PMR Pro) that was almost identical and targeted the same countries.
I had many long and detailed discussions with the Google Dev Support over several weeks, in which I disclosed all my relevant apps (screenshots, content, subject matter of the apps). Now I am able to conclude with a high degree of certainty what I explained before. As I said, my apps were all re-approved and have been back in the Play Store for for several months without any further violation of the “Repetitive Content” policy.
Note: You have to pay attention to this urgently, otherwise the work of years (and in my case also a lot of money) can be lost overnight without prior warning.
And it’s absolutely uncertain whether others will be as persistent as I was or be as lucky that Google listened to my argumentation for so long and ended up following it.
I hope I can help people to fix / avoid these problems, which have already affected well-known apps with millions of users.
And yes google policy is strange they also removed our India’s biggest payment app without any prior warning (Paytm) and then after some days it reinstate again.