Guix as a package manager knows how to build software, and how to manage all the files. If you build the right software, plus a few more things with some files, then you end up with something that'll boot.
One really neat thing about this is that this is set out in one Git repository, making it pretty easy to reason about.
> Is this HURD? Or does it have a kernel? Is it some sort of hypervisor or application container? I always found HURD to be exciting in a sci-fi kind of way but from what I’ve heard this sounds like it’s actually something that could be practically useful. Where would I start? Does it make any sense running in VirtualBox let’s say?
Guix by default uses Linux-libre, but you can run it with the Hurd.
You can start by using Guix as a package manager, it'll fit alongside your existing operating system (providing it's something using Linux). There's also a VM image you can download from the website if that's of interest.
It has some nice properties such as not needing root privileges to install things, atomic upgrades and rollbacks, reproducible builds, multiple or transient (optionally containerized) "environments" (collections of packages), etc.
Guix System is a full distro built around the package manager. It gives you atomic upgrades and roll-backs for the entire system (you can choose any previous "generation" in the boot loader).