Access data from other apps? e.g. Calibre Web and Syncthing


how can I access files from other apps?

For example: I have installed Calibre Web and Syncthing, for syncing all my ebooks from my notebook
to umbrel. In Syncthing all files were sent to /var/syncthing/Calibre.
But I cannot find the folder in Calibre Web? Also I cannot find the book folder of Calibre Web in Syncthing.

Thank you very much for help.