In Debians, it's /etc/X11/default-display-manager.
In openSUSE 15.0, it's /etc/alternatives/default-xsession.desktop.
startx opens TDE session.

Neither works in Fedora 29.
startx starts X, then exits.
startx /opt/trinity/bin/starttde works, but shouldn't be needed. There are only
two installed, Trinity, and IceWM. Default should not be neither.
-- 
Evolution as taught in public schools is religion, not science.

 Team OS/2 ** Reg. Linux User #211409 ** a11y rocks!

Felix Miata  ***  http://fm.no-ip.com/