How to configure Xinerama?

I have a 5k monitor (LG Ultrafine 5k) that seems to require Xinerama in order to work well in KDE Plasma.

How do I enable Xinerama when using Clear Linux? More generally how do you edit X configuration?

probably similar to other distros, except config files may be missing or in subdirectories of /usr/share/ see stateless — Documentation for Clear Linux* project
you may add config files to the appropriate place (over here i beleive to /etc/X11/xorg.conf.d/ - that is where a keyboard.conf file was added when i changed the x11 keymap with localectl)