So, looking at a GNOME project I know a little about, Rhythmbox...

* Source code is: https://gitlab.gnome.org/GNOME/rhythmbox/-/tree/master

* Ways to work on it: https://gitlab.gnome.org/GNOME/rhythmbox/-/blob/master/HACKI...

* How to build it locally: https://gitlab.gnome.org/GNOME/rhythmbox/-/blob/master/INSTA...

What GNOME projects are you looking at?