Combining and grouping arrays using JavaScript

I have two sets of data that need to be combined and grouped a certain way.

First set of data:

let overallCert =
[
  {
      "id": "1",
      "entity_id": "3",
      "status": "Certified",
  },
  {
      "id": "379",
      "entity_id": "417",
      "status": "Certified",
  }
];

Second set of data:

let userCerts =
[
  {
      "id": "640",
      "entity_id": "417",
      "method": "Field Study",
      "strand": "",
      "date_completed": "2016-07-15T08:00:00.000Z"
  },
  {
      "id": "814",
      "entity_id": "417",
      "method": "Field Study",
      "date_completed": "2019-07-15T08:00:00.000Z"
  },
  {
      "id": "844",
      "entity_id": "3",
      "method": "Online",
      "date_completed": "2022-03-28T08:00:00.000Z"
  },
  {
      "id": "845",
      "entity_id": "3",
      "method": "Field Study",
      "date_completed": "2022-03-28T08:00:00.000Z"
  }
];

I want to merge and group these arrays of objects by entity_id to create this output below…

Desired output:

let desiredOutput = 
[
  [
    [
      {
        "id": "640",
        "entity_id": "417",
        "method": "Field Study",
        "date_completed": "2016-07-15T08:00:00.000Z"
      },
      {
        "id": "814",
        "entity_id": "417",
        "method": "Field Study",
        "date_completed": "2019-07-15T08:00:00.000Z"
      },
    ],
    [
      {
        "id": "379",
        "entity_id": "417",
        "status": "Certified",
      }
    ]
  ],
  [
    [
      {
        "id": "844",
        "entity_id": "3",
        "method": "Online",
        "date_completed": "2022-03-28T08:00:00.000Z"
      },
      {
        "id": "845",
        "entity_id": "3",
        "method": "Field Study",
        "date_completed": "2022-03-28T08:00:00.000Z"
      }
    ],
    [
      {
        "id": "379",
        "entity_id": "417",
        "status": "Certified",
      }
    ]
  ]
];

So far, I have managed this:

let certsDataCombined = overallCert.map(item => ({ ...item,
  0 : userCerts.filter(c => c.entity_id == item.entity_id)
}));

let userCertsGroupBy = groupBy(certsDataCombined, "entity_id");

function groupBy(arr, prop) {
  const map = new Map(Array.from(arr, obj => [obj[prop], []]));
  arr.forEach(obj => map.get(obj[prop]).push(obj));
  return Array.from(map.values());
}

This code almost does the trick, but I need to encapsulate the overallCert data in its own array, plus the nesting is a little off. Here is the current output: