Mehrere PDFs in Unterordner zu einer mit Inhaltsverzeichnis zusammen führen

This is very tricky to do. You can merge them so that the merged file has bookmarks pointing to the first page of each file, and then use a script to convert those bookmarks to a clickable TOC (I've developed a paid-for tool that allows you to do just that: https://www.try67.com/tool/acrobat-create-a-table-of-contents-toc-in-a-pdf-file-from-bookmarks), but the structure of the folders will be lost, and all the bookmarks will appear at the same level.

If you want to maintain this structure in the bookmarks (and later on the TOC) a much more complex tool will be required.