this one works!

apps/git/home.nix

@@ -2,9 +2,9 @@
   programs.git = {
     enable = true;
     extraConfig = {
-      user.name = "${myConfig.nixos.Git.user}";
+      user.name = "${myConfig.git.username}";
-      user.email = "${myConfig.nixos.Git.email}";
+      user.email = "${myConfig.git.email}";
-      init.defaultBranch = "${myConfig.nixos.Git.defaultBranch}";
+      init.defaultBranch = "${myConfig.git.defaultBranch}";
     };
   };
 }

hosts/hakase/configuration.nix

@@ -1,6 +1,12 @@
-{pkgs, ...}: {
+{
+  pkgs,
+  myConfig,
+  ...
+}: {
   imports = [
     ./hardware-configuration.nix
     ../../modules/system/default.nix
   ];
+
+  programs.ssh.extraConfig = myConfig.ssh.extraConfig;
 }