.xinitrc will be in your home directory (and its hidden)

There might be one or more lines in it, or it may be blank/not exist. basically, whatever is in it will be run when you run the command startx from text mode.

so if it had

startkde

in the /home/knoppix/.xinitrc file, then if the user knoppix was in text mode and ran the command startx, KDE would be started. Each WM/DE will have its own launch command for .xinitrc. You can have several in the file so long as all but the one you are using have a "#" at the beginning so they will be ignored. See an example below-

startkde
#/usr/bin/icewm
#/usr/bin/fluxbox
#exec gnome-session