The construct in the opening post isn’t supported. You need to look into nested sections in the docs.
You obviously want them as seen in your example. A quick tip is to rename “index.md” => “_index.md” – but please read and understand the docs. Open another more specific thread if you have follow-up questions.
I think I understood my problem:
My _default/list.html was empty.
However it is used to generate branch pages.
So I just copied single.html to list.html as they should be the same for me.
It is working now.