commands to determine your login manager / greeter
your session
loginctl list-sessions
session info and service (display manager)
`loginctl show-session <session_number> --all
the display manager is what handles the greeter eg: lightdm
You can set the default greeter by changing the [Seat:*] section of the LightDM configuration file, like so:
/etc/lightdm/lightdm.conf
[Seat:*]
...
greeter-session=lightdm-yourgreeter-greeter
...