I don't think it's possible to just "wing it" by copy-and-paste.
2.
I did not understand the introductory paragraph about how services "extend" each other. Does every service have hooks for possible extensions? What if a new service doesn't fit existing extension hooks?
(I can understand service dependencies of course, but it seems to go beyond that.)
This is less flexible-by-default than NixOS module, where any module can modify any other module. That is by design. The Guix developers see NixOS's approach as failing the principle-of-least-authority, where any arbitrary module (even those imported via flakes) can add a root SSH key.
1. It seems one needs to know some Scheme in order to write these files:
https://www.scheme.org/
I don't think it's possible to just "wing it" by copy-and-paste.
2. I did not understand the introductory paragraph about how services "extend" each other. Does every service have hooks for possible extensions? What if a new service doesn't fit existing extension hooks?
(I can understand service dependencies of course, but it seems to go beyond that.)