https://nixos.org/nixos/options.html
https://search.nixos.org/options

samples
$ nixos-option system.build | grep -o 'installBootLoader = [^ ]*'

2024
LibrePhoenix - Custom Options and If-else Statements in Your NixOS Config 0:00 of 26:15
    https://www.youtube.com/watch?v=Qull6TMQm4Q
    https://librephoenix.com/2023-12-26-nixos-conditional-config-and-custom-options.html