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Additional fields that have been transformed can be queried in MongoDB using the $project operator.

The $project operator is used to specify the fields to be returned in the query result. It can also be used to rename, transform, or add new fields to the result.

Here is an example:

Suppose we have a collection called "users" with documents that look like this:

{
  "_id": ObjectId("6072b11a8ebb614a21a747c0"),
  "name": "John Smith",
  "age": 35,
  "address": {
    "street": "123 Main St",
    "city": "New York",
    "state": "NY"
  }
}

If we want to add a new field called "full_address" that concatenates the street, city, and state fields, we can use the following query:

db.users.aggregate([
  {
    $project: {
      name: 1,
      age: 1,
      full_address: { $concat: [ "$address.street", ", ", "$address.city", ", ", "$address.state" ] }
    }
  }
])

The result will look like this:

{
  "_id": ObjectId("6072b11a8ebb614a21a747c0"),
  "name": "John Smith",
  "age": 35,
  "full_address": "123 Main St, New York, NY"
}

In this example, we use the $project operator to specify the fields "name", "age", and "fulladdress" to be returned in the result. We also use the $concat operator to concatenate the "address.street", "address.city", and "address.state" fields into a new field called "fulladdress".