First we need to check the masterId at each level
if its equal at each level than we need to check for ref,
if ref is equal than add the quantity of both unique ref.

let obj1 = {
  "obj": [
    {
      "name": "abc",
      "obj": [
        {
          "name": "def",
          "obj": [
            {
              "name": "ghi",
              "data": [
                {
                  "quantity": 1,
                  "ref": "ABCEFG"
                },
                {
                  "quantity": 1,
                  "ref": "HIJKLM"
                  },
                {
                  "quantity": 1,
                  "ref": "ABCDEF"
                }
              ],
              "masterId": "07a5b470"
            }
          ],
          "masterId": "07a5b470"
        }
      ],
      "masterId": "07a5b470"
    }
  ]
}

let obj2 = {
  "obj": [
    {
      "name": "abc",
      "obj": [
        {
          "name": "def",
          "obj": [
            {
              "name": "ghi",
              "data": [
                {
                  "quantity": 1,
                  "ref": "ABCEFG"
                },
                {
                  "quantity": 1,
                  "ref": "HIJKLM"
                },
                {
                  "quantity": 1,
                  "ref": "XYZ123"
                }
              ],
              "masterId": "07a5b470"
            }
          ],
          "masterId": "07a5b470"
        }
      ],
      "masterId": "07a5b470"
    }
  ]
}

Expected Output:

{
  "obj": [
    {
      "name": "abc",
      "obj": [
        {
          "name": "def",
          "obj": [
            {
              "name": "ghi",
              "data": [
                {
                  "quantity": 2,
                  "ref": "ABCEFG"
                },
                {
                  "quantity": 2,
                  "ref": "HIJKLM"
                },
                {
                  "quantity": 1,
                  "ref": "ABCDEF"
                },
                 {
                  "quantity": 1,
                  "ref": "XYZ123"
                }
              ],
              "masterId": "07a5b470"
            }
          ],
          "masterId": "07a5b470"
        }
      ],
      "masterId": "07a5b470"
    }
  ]
}