elementary uses gdm?
You want lightdm!!
if GDM then
sudo systemctl disabled gdm.service (this will shut down the old login Manager )
sudo systemctl enable lightdm.service (this will include new)
something recommend putting arch hands and more of these questions You will have good luck!!
if You read this myubuntu.ru/rukovodstvo/kak-vybrat-i-kak-izmenit-d...
the article is a bit outdated view https://ru.wikipedia.org/wiki/Systemd