NuPhy Air75 fn + h j k l to arrow keys with Karabiner element complex modifications.

air75-vim-arrows.json

{
  "title": "VIM arrows",
  "rules": [
    {
      "description": "fn + h j k l to arrow keys",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "key_code": "h",
            "modifiers": {
              "mandatory": ["fn"],
              "optional": ["control", "option", "command", "shift"]
            }
          },
          "to": [
            {
              "key_code": "left_arrow"
            }
          ]
        },
        {
          "type": "basic",
          "from": {
            "key_code": "j",
            "modifiers": {
              "mandatory": ["fn"],
              "optional": ["control", "option", "command", "shift"]
            }
          },
          "to": [
            {
              "key_code": "down_arrow"
            }
          ]
        },
        {
          "type": "basic",
          "from": {
            "key_code": "k",
            "modifiers": {
              "mandatory": ["fn"],
              "optional": ["control", "option", "command", "shift"]
            }
          },
          "to": [
            {
              "key_code": "up_arrow"
            }
          ]
        },
        {
          "type": "basic",
          "from": {
            "key_code": "l",
            "modifiers": {
              "mandatory": ["fn"],
              "optional": ["control", "option", "command", "shift"]
            }
          },
          "to": [
            {
              "key_code": "right_arrow"
            }
          ]
        }
      ]
    }
  ]
}

1. download Karabiner Elements if not yet installed
2. download air75-vim-arrows.json
3. Put the file in ~/.config/karabiner/assets/complex_modification
4. Open Karabiner element and enable the modification
5. Done!