If SDDM is the default display manager in Tumbleweed, why is sddm disabled in the services?
Thank you,
Loren
Because in (open)SUSE, the display manager is started by a generic display-manager.service.
This reads the configuration from /etc/sysconfig/displaymanager and runs the dm specified in DISPLAYMANAGER.
Also,
SDDM is the preferred Display Manager only when you’re running KDE.
If you’re running something else, you won’t have SDDM enabled.
TSU
Well, it’s not really about what you’re running, but rather what you installed. 
On a default KDE installation sddm is installed/used, a default GNOME installation uses gdm, others probably use lightdm.
The display manager is completely independent of the desktop though, you can use any DM you like.
I think also how and if you upgraded from a previous version to TW or Leap. kdm may stick
Yes, the configuration is not changed on updates/upgrades.
If you uninstall the configured display manager, the system (i.e. display-manager.service) will use xdm as fallback.