Whenever I setup arch I use xorg-xinit and its .xinitrc file to startup my Desktop Environment. I am using Ubuntu on my laptop with the amazon stuff turned off, however, I have noticed there is no .xinitrc file in my home folder.
How do distros that dont use .xinitrc start their desktop environment on boot?
Ubuntu uses systemd as of 15.04, the dm is probably launched there. type systemctl into the terminal and you will see the services it has enabled, you can probably disable gdm with: sudo systemctl disable gdm.service
then reboot and it should leave you at a tty.. As for starting the wm without the dm in that instance..? I've no clue, havent used ubuntu in ages, you could make an .xinitrc and just enter exec 'w/e the name for unity is to launch' and try startx.
if that doesnt work just re-enable the gdm service using sudo systemctl enable gdm.service
your .xinitrc is basically the equivalent of a dm. You can write it to load different desktops on command, with full proper support from gnome, kde etc with no need for a graphical login. I used this when running Xmonad.
login managers (as well as other ways) keep default login boot scripts somewhere in /etc .. i think you should be able to find something about it online.