Now I imagine that /sdb/0023/dwonloads is the actual catalogue to which I download on my remote seedbox. WHat I don’t know is how to properly map it and enable downloading to my local server?
You will need to mount a share from the seedbox locally (SSHFS or similar) and then use Sonarr’s Remote Path Mapping to map the remote path reported by your download client to a local path available on the machine running Sonarr.