Login or display manager is an interface for users to log in to their Linux system. Different Linux distributions choose other login managers for the system, and you can install and configure display managers on your own.
Ubuntu's default display or login manager is GDM, while Kubuntu uses SDDM. Some other display managers for Linux are XDM and LXDM.
If you install a new login manager using apt, the installation script will prompt an option for you to choose which login manager to use for system login. If you already have multiple display managers installed, you can use dpkg-reconfigure to select the login manager to use.
$ sudo dpkg-reconfigure gdm3 [sudo] password for user:
Use <tab> key to navigate to the Ok button and press [ENTER] if you're first presented with the following screen. You'll get this screen if you're using a low resolution monitor and/or not running terminal in full screen.
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