Hi
The question was: How do I get (i.e.) Transmission to download files into my NAS’ shared network-folder?
Can I set this via Dietpi-Drive-manager?
Or do I have to change the transmission-config-file?
You can setup/mount the NAS (NFS/Samba/CIFS at least) via dietpi-drive_manager. But you need to change the transmission config the download into that mount or move completed downloads there, which probably makes more sense to avoid doubled network use when relaying downloaded packets directly to the NAS.
DietPi-Drive_Manager automatically mounts the network drives with permissions that allow R/W access to provided media software btw.
If you use some media organisation tool like Sonarr/Radarr/Lidarr it is probably best to let download to the Pi and have those tools sort the finished downloads into your NAS.