The structure is like this:

  - gnome-session-wayland.target, gnome-session-x11.target: Top-level
    targets launched directly to start the regular GNOME (Shell) session
    for wayland or x11 respectively.
  - gnome-session@.service: Execute gnome-session-binary tself for the
    specified (template) session. Is active once
    org.gnome.SessionManager is claimed on the bus.
  - gnome-session-bus.target: Bound to gnome-session@.service - if
    something needs to start after any instance of gnome-session@
    starts, it can be After= this one. gnome-settings-daemon uses this.

We also split out gnome-wayland.desktop.in, as it is going to need to
launch gnome-session-wayland.target.