Mongodb populating field on aggregation query

What you have done is correct. After the lookup between Product and Comment, you need to have another lookup to join User also.

You need to add following stages to achieve your target

• unwind to unstructured the comments[] array
• lookup to join User collection
• Since lookup provides an array, we can get the first element of the array using $arrayElemAt with the help of safety operator $ifNull. (If the comments [] is empty, then the script throws error. To handle that we use $ifNull)
• We already unstructured the array, group helps to regroup it

The stages are given below

  {
    $unwind: "$comments"
  },
  {
    $lookup: {
      from: "User",
      let: {
        id: "$comments.commented_by"
      },
      pipeline: [
        {
          $match: {
            $expr: {
              $eq: [
                "$_id",
                "$$id"
              ]
            }
          }
        },
        
      ],
      as: "comments.commented_by"
    }
  },
  {
    $addFields: {
      "comments.commented_by": {
        $ifNull: [
          {
            $arrayElemAt: [
              "$comments.commented_by",
              0
            ]
          },
          {}
        ]
      }
    }
  },
  {
    $group: {
      _id: "$_id",
      title: {
        $first: "$title"
      },
      hasCommented: {
        $first: "$hasCommented"
      },
      comments: {
        $push: "$comments"
      }
    }
  }

Working Mongo playground

Note : FYI,The variable and collection name may be different.