neovim-flake/docs/manual/hacking/additional-plugins.md

Adding Plugins

To add a new neovim plugin, first add the source url in the inputs section of flake.nix

{
  inputs = {
    # ...
    neodev-nvim = {
      url = "github:folke/neodev.nvim";
      flake = false;
    };
    # ...
  };
}

Then add the name of the plugin into the availablePlugins variable in lib/types/plugins.nix:

# ...
availablePlugins = [
  # ...
  "neodev-nvim"
];

You can now reference this plugin using its string name:

config.vim.startPlugins = ["neodev-nvim"];