You can perhaps have a look at how the kernel configfile is generated, or you can build that and compare it with your own options. For example:

$ nix-build -A linux_xanmod.configfile
$ $EDITOR ./result

been ages since I looked at the nixpkgs stuff around kernel config but I'm pretty sure we rely on upstreams defaults, and common-config.nix just adds more.

`nix log nixpkgs#linuxKernel.kernels.<kernel_version>.configfile```

will show you the configfile build log.

i.e:

*** Default configuration is based on 'x86_64_defconfig'
#
# configuration written to .config
#