Simple Desktop Display Manager (SDDM) is a cross-desktop X11 and Wayland display manager. It was built to be modern, fast, simple, beautiful, and highly customizable. It is based on Qt and its markup language QML and has customizable looks using themes. It is the standard display manager of the Qt-based desktop environments KDE and LXQt. It is present in Debian stable since stretch (Debian 9).
apt install sddm
or search SDDM in your favorite graphical APT frontend like e.g. Synaptic and install it from there.
SDDM configuration files can be placed at /usr/lib/sddm/sddm.conf.d or /etc/sddm.conf.d
Alternatively, a file named sddm.conf in /etc/ can be used.
The man page for sddm.conf contains a comprehensive list of options.
To change the current default Display Manager, run
Create some text file in one of the above folders, e.g. /usr/lib/sddm/sddm.conf.d/autologin with the following content, customizing the ALL-UPPERCASE parts:
[Autologin] User=USERNAME Session=DENAME.desktop Relogin=
Put the user name of the user to be logged in automatically after User=.
As Session= name use the name of one of the files found under /usr/share/xsessions/, e.g. plasma.desktop for KDE or lxqt.desktop for LXQt.
Relogin= may be set to true in order to immediately again autologin after logoff, if false or omitted/left blank, SDDM will autologin only for first login after boot.