Join two collection in mongodb

I'm new in mongodb. Could you please tell me how to perform join operation in this. I've two collection:

Collection 1 ("user")

{
 _id: "d04d53dc-fb88-433e-a1c5-dd41a68d7655",
 userName: "XYZ User",
 age: 12
}

Collection 2 ("square")

{
 _id: "ef6f6ac2-a08a-4f68-a63c-0b4a70285427",
 userId: "d04d53dc-fb88-433e-a1c5-dd41a68d7655",
 side: 4,
 area: 16
}

Now I want to retrieve the data from collection 2 is like this. Expected output:

{
 _id: "ef6f6ac2-a08a-4f68-a63c-0b4a70285427",
 userId: "d04d53dc-fb88-433e-a1c5-dd41a68d7655",
 userName: "XYZ User",
 side: 4,
 area: 16
}

Thanks in advance :)

Here's one way to do it.

db.square.aggregate([
  {
    "$lookup": {
      "from": "user",
      "localField": "userId",
      "foreignField": "_id",
      "as": "userDoc"
    }
  },
  {
    "$set": {
      "userName": {
        "$first": "$userDoc.userName"
      }
    }
  },
  { "$unset": "userDoc" }
])

Try it on mongoplayground.net.