It’s system-wide (unless home-manager is involved).
They’re a part of the immutable install, whose components reside in /nix/store
and are symlinked to /etc
.
Example from my computer:
$ realpath /etc/sddm.conf
/nix/store/slkq2k8vc4rx4ag55zf8ssl7qd9ry49v-sddm.conf
nvidia-settings and https://github.com/Lurkki14/tuxclocker support fan control on Wayland