Although flying well under the radar of the average Linux user, D-Bus has been an integral part of Linux distributions for nearly two decades and counting. Rather than using faster point-to-point interprocess communication via a Unix socket or such, an IPC bus allows for IP communication in a bus-like manner for convenience reasons. D-Bus replaced a few existing IPC buses in the Gnome and KDE desktop environments and became since that time the de-facto standard. Which isn’t to say that D-Bus is well-designed or devoid of flaws, hence attracting the ire of people like [Vaxry] who recently wrote an article on why D-Bus should die and proposes using hyprwire instead.
The broader context is provided by [Brodie Robertson], whose video adds interesting details, such as that Arch Linux wrote its own D-Bus implementation rather than use the reference one. Then there’s CVE-2018-19358 pertaining to the security risk of using an unlocked keyring on D-Bus, as any application on said bus can read the contents. The response by the Gnome developers responsible for D-Bus was very Wayland-like in that they dismissed the CVE as ‘works as designed’.
Itvery annoying also that you need some weird systemd command incantations to make it detectable by environment variables when using Wayland compositors X11 window managers that aren't full desktop environments, in combination with XDG portals.
I struggled so much getting dark mode running well with darkman. And screen sharing with Niri. Hhhrg.
Pretty sure the systemd command you're talking about just adds the bus to PATH. You very likely could just do that yourself.