No worries.
To make your life easier you will want to pass the same "volume" to each of your containers so that they are all able to interact with the files the same way. For instance, if your movies are in /home/username/media/movies then make a volume for radarr, you can name it anything but for this example I'll use data, like so in docker:
/home/username/media:/data
Then inside radarr you can make your path inside.media management, root folders:
/data/movies
It works the same way for your downloads, just make sure your downloads go somewhere in the media folder, eg. /home/username/media/downloads. Then for your download client, use /home/username/media:/data in docker and inside the client download to /data/downloads.
Hope that makes sense
They are not secure, unless you are using the secure message feature (if the person has a matrix account linked to their Lemmy account the option will be there and you can message them through matrix).