The pre-installed firmware on the Quadra is based on Debian 12, kernel 6.6.y.  At its core, it is built using Armbian.

To learn more about building your own image, see the article Building Amrbian from source in the Projects category.

You can download our current firmware here. To flash it, see this article.

If you'd prefer to start with vanilla Armbian/Debian, you can download those images here.