buildLinux details

Hi, I am new to Nix universe. I am trying to cross compile Linux kernel for a riscv64 machine. I am referring https://github.com/zhaofengli/nixos-riscv64 How can I find detail about buildLinux() there is no information about it in Nix Manual. Basically I want to pass a custom config file as a default configuration for kernel compilation.

It is briefly mentioned here:

It is NixOS-specific, hence it is in the NixOS-manual, not in the Nix-manual.