It’s an unofficial open-source daemon used by alternative Spotify clients. I used it once for a terminal Spotify client. It’s a pretty neat piece of software.

The best information I can find on getting Guix on the Raspberry Pi is the issues page for nonguix https://gitlab.com/nonguix/nonguix/-/issues/128

It’s based on the stats found here. https://repology.org/repositories/statistics/total

Take a look at this Guix blog post. https://guix.gnu.org/en/blog/2022/keeping-ones-home-tidy/