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.