To amplify RedWeasel’s very good answer, fstab
runs as root and unless you specify otherwise, the share will mount with root as the owner on the local machine. From the perspective of the Samba server, it’s the Jellyfin user accessing the files, but on the local machine, but local permissions come into play as well. That’s why you can get at the files when you connect to the share from Dolphin in your KDE system—it’s your own user that’s mounting the share locally.
Linux runs fine on Intel Macs. There are a couple peculiarities you’ll want to be aware of, though.
Other than those initial hiccups, everything works pretty flawlessly.