I closed everything that was using more ram that plasmashell on my machine and ended up with about 3.3GB of memory usage (down from 12ish, mostly firefox, but evolution and MS teams were also pretty ram hungry).
Notable not-kde things that were using significant ram after doing this:
Various pipeware processes, adding up to at least 100MB
systemd-journald, also about 100MB
Several things enabled to run gnome applications (dconf-service, xdg-desktop-portal-gnome, goa-daemon) also about 100MB total
One thing I noticed about plasma is how many processes there are; plasmashell is only about 10% of the usage of a plasma system doing "nothing" and it's the single largest process.
Interesting, I'm using KDE Plasma as well, and with only a terminal open, my RAM usage is about 1.25GB, and < 3GB with Firefox opened (I don't open a lot of tabs though).
Notable not-kde things that were using significant ram after doing this:
Various pipeware processes, adding up to at least 100MB
systemd-journald, also about 100MB
Several things enabled to run gnome applications (dconf-service, xdg-desktop-portal-gnome, goa-daemon) also about 100MB total
One thing I noticed about plasma is how many processes there are; plasmashell is only about 10% of the usage of a plasma system doing "nothing" and it's the single largest process.