[Nix] allows users to reliably reproduce the same state coming from any other state, which is also used for safe rollbacks.

What does that mean for configuration? How does configuration and customization play into Nix declarations?

I'm thinking of what would usually be in /etc/, and what classic package managers like apt/dpkg would deliver a default of but not replace if already existing. Where you make your specific program and service configuration that is independent of the theoretically immutable package program data.