The simplest way is to get the gluetun app (on the BigBearCasaOS app store), get it set up with your VPN, then in qbittorrent do this (with the port):

Image.

You can do the same thing on Prowlarr in the “proxy” settings. Radarr and Sonarr have a similar Proxy setting but you don’t really need to put those behind the VPN there.