NVIDIA

Card type

MXM / output-providing card (shows as VGA Controller in lspci), i.e. graphics card in desktop computer or in some laptops
muxless/non-MXM Optimus cards have no display outputs and show as 3D Controller in lspci output, seen in most modern consumer laptops

Non-optimus mode

You need MXM card. Follow NVIDIA Graphics Cards section in official manual.

In case of laptop you may also need to use BIOS option to select which card to use for internal display.

Optimus

There are currently two solutions available under NixOS:

Bumblebee

Unofficial solution.

Use

hardware.bumblebee.enable = true;

option.

Pros: out of box experience, just start the game via primusrun or optirun wrapper, works under existing X11 server

Cons: some performance penalty, no Vulkan support, things like CUDA and OpenCL are problematic to configure

Nvidia PRIME

You need muxless card. Official solution by nvidia.

1. Configure NixOS to allow Intel and Nvidia X.Org drivers coexist together (only needed because NixOS forces LD_LIBRARY_PATH variable propagation), and also to disable nvidia card by default.

/etc/nixos/configuration.nix

{
  # disable card with bbswitch by default
  hardware.nvidiaOptimus.disable = true;
  # install nvidia drivers in addition to intel one
  hardware.opengl.extraPackages = [ pkgs.linuxPackages.nvidia_x11.out ];
  hardware.opengl.extraPackages32 = [ pkgs_i686.linuxPackages.nvidia_x11.out ];
}

2. Download primerun script.

Like with Bumblebee, start the game via primerun.sh.

Pros: better performance, Vulkan is supported. CUDA and OpenCL should work, though CUDA needs an additional device creation rule https://github.com/NixOS/nixpkgs/blob/05e375d7103ac51e2da917965c37246c99f1ae4f/nixos/modules/hardware/video/nvidia.nix#L72

Cons: there are still some rough edges noted on the top of script, but otherwise it works fine for me, starts new X11 server

non-NixOS case

GL drivers on non-NixOS: The nixGL project provides wrapper to use GL drivers outside of NixOS. You need to have nvidia drivers installed on your distro (for kernel modules). Then supply nvidia driver version you use on host system to nixGL.
Optimus only. Primerun will build nvidia kernel modules against your currently running kernel.