How to create an array entry with ObjectId and Number

I am new to mongodb and am trying to make this type of entry:

{
    "resources": [
        {
            "amount": 1,
            "resource": {
                "_id": "61be82b9549b4ede0c8df07e"
            }
        }
    ]
}

Here is my schema code:

const schema = new Schema({
  resources: [
    {
      amount: {
        type: Number,
        required: true,
      },
      resource: {
        _id: {
          type: Schema.Types.ObjectId,
          ref: "Resource"
        }
      }
    }
  ]
});

here is the json code i send

{
    "resources": [
        {
            "amount": 1,
            "resource": "61be82b9549b4ede0c8df07e"
            
        }
    ]
}

After processing the request, the following entry is created

{
    "resources": [
        {
            "amount": 1,
            "resource": "61be82b9549b4ede0c8df07e",
            "_id": "61ebebf0e20286eee987bdc3"
        }
    ],
    "_id": "61ebebf0e20286eee987bdc2",
    "__v": 0
}

Id for the resource was created correctly, but I can’t figure out where the resources._id key came from? Where did I make a mistake?