KDE Plasma on X offers the option to use a differnt window manager than the default KWin. This way one can use many of the integrations KDE offers, but with for example the i3 window manager to have proper tiling support. This only works on X, not on Wayland.
With Plasma 5.21 it is now possible to "boot" KDE using systemd user services; that is, KDE Plasma provides a bunch of unit files that get started atfer you login in your graphical user session. Inidividual components, such as KWin, have individual unit files, and dependencies are specified using the usual systemd relationships between units. Among other things this allows for better ressource control using automatically created CGroups.
More details here: https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/47
Sadly this breaks the way one could change the window manager, that was used until now. I noticed af