The external symlinks are mostly here to be able to expose the right targets and services aliases to systemd to allow the proper startup of a KDE Plasma session.
The snap is not intended for wide use by many snaps: it is intended to be only used by the plasma-desktop-session snap and maybe a future sddm snap.
we currently can’t express or import systemd targets, so this content snap is coming with them
said targets refer to services with well known names
snapd will generate service files using its own naming rules (services described in the plasma-desktop-session snap)
so in the content snap we also provide symlinks since it’s the way to provided alias names to systemd (this helps reconcile between targets and snapd generated services)