The work done is quite simple. They take the Ubuntu version, passe it under a
filter of non-free software (men work) and then change the packages that they
want to change.
So as a short example. Trisquel 6.0 will be Ubuntu 12.04 but without non-free
content (firmware, drivers, software etc..) and with only a few software
modified (like Gnome with small changes in order to have the wonderful DE
that we have today in all of our Trisquel Installations by default, and the
personalized gnome-control-center etc...).
Some packages may come from Debian directly but only in last case.
You can see more information here:
https://trisquel.info/en/wiki/how-trisquel-made
