There shouldn't be much difference between understanding nix and guix, it is basically the same idea.
Guix is a known language scheme.
nix is off - I can't work out what parameters are needed for any one function call they all appear to be in a global namespace.
Unfortunately I need to be able to use non Linux systems
If you are talking about something you saw in the nixpkgs repo, then that is probably the callPackage design pattern [0], which is basically a form of automatic dependency injection.
It definitely made it hard for me to grok how nixpkgs package definitions worked at first - there was so much stuff getting injected dynamically, without any trace of where those dependencies lived.
The callPackage pattern is an important piece that allows you to override a dependency with a different version, though, which is essential when customizing existing packages for your own needs.
[0]: https://nixos.org/guides/nix-pills/callpackage-design-patter...
How files all each other and the parameters is odd. You call a function with two parameters but the file starts with many