You are most likely using LightDM (lightdm package) as your login manager (as @Daniel T said) instead of the default GDM (gdm3 package). I suppose that you ended up with LightDM either because you didn't do a clean install, rather you upgraded from another installation where you had lightdm installed, or because you followed some guide to configure your system and you installed lightdm without noticing.
To remove lightdm and replace it with gdm3 run the following commands in a terminal:
sudo apt install gdm3
sudo dpkg-reconfigure gdm3
systemctl stop lightdm
sudo apt remove lightdm
systemctl start gdm3