The Trisquel project mainly is about integrating pieces, the packages
(sometimes after small modifications, through the package helpers). Trisquel
does not develop the software you run. That software is written in a variety
of different programming languages.
Here is "how Trisquel is made":
https://trisquel.info/en/wiki/how-trisquel-made
You can contribute to the Trisquel project:
https://devel.trisquel.info/trisquel/package-helpers/blob/belenos/CONTRIBUTING.md
Or you can contribute to your favorite program, whatever it is.
To learn git (the basics is enough to start contributing):
https://git-scm.com/documentation
