In a new installation of Kubuntu (on a btrfs partition) I got a root folder with only two folders
@ and @home.
It is a mistake?
Sounds like a Kubuntu problem, not a KDE problem. You might have better luck asking in an Ubuntu-specific forum.
Those are not folders, they are the root and home subvolumes, they should be mounted as such in your fstab.
The reason you see them is because you have the partition mounted, not the @ (root) subvolume.
If the installation did not do this for you, I agree with above, ask in a Kubuntu forum.
I see, thank you! I will search in Kubuntu forums.