What I want to achieve is transferring my mp3 files to The TrueNas server
I've never touched Jellyfin, and I've only played with SCALE a little bit, but what part of this is a problem? "Create share, copy files to it" seems pretty straightforward*. So does "put them on an external drive and use the
Import Disk feature to copy them over." Where are you having problems with that? I'm assuming you have some familiarity with what a NAS is and how it works, since you installed one.
What are the first steps to get the files in a folder structure?
I guess the first step would be to create the folders you want, and the second would be to put the files there. But, as with the last question, this is obvious enough that I don't think it's your real question--though from what you wrote, I have no idea what the real question is.
Then Install Jellyfin APP
Any of the TrueCharts guides would be relevant here, as all of their apps work pretty much the same way. You'd want to add storage for the app, pointing to the directory where you stored the MP3s; this guide should help with that:
We provide the option to mount as many host folders as you want.
truecharts.org
ant create media libraries on this folder where the mp3 are
I'm sure the Jellyfin docs would help here.
It really sounds like you're looking for a guide that starts at "freshly-installed SCALE server with no users, pools, or shares", and ends with "Jellyfin serving my media." I don't think you're going to find such a guide, and I doubt anyone's going to be very motivated to write one; I'm not aware of one for Plex or Emby either. But there should be ample support available for each part of this.
*I'm aware that the TrueCharts folks advise against using shared directories as external storage for their containers due to issues with ACLs, but I don't see in their docs what they recommend as an alternative.