simple question, where are the persistent but mutable configurations and save files of modules like Jackett stored?
services.jackett.enable = true;
For a user application, configuration would be in ~/.config/packagename and state would be in ~/.local/share/packagename.
I found the directory, it is in /var/lib/jackett/.config/Jackett/
Several services have a dedicated option for it: services.jackett.dataDir . Sadly not all do but in this case you are lucky.
true, and there (search.nixos.org) I would have also found the default directory
Very often this is implicit and not inspectable from the nixos option search. In that case I’d recommend checking the systemd unit with systemctl cat foo.service and look for StateDirectory=, which allocates paths below /var/lib automatically.