await Response.find({
      responder_id: req.user._id,
    })
      .populate({ path: "query_id", select: ["query_type", "content"] })
      .find({ slider_value: 761 })
      .find({ query_id: {} });

When the code above is runned, this is the error message returned to Postman:

{
    "stringValue": "\"{}\"",
    "valueType": "Object",
    "kind": "ObjectId",
    "value": {},
    "path": "query_id",
    "reason": {},
    "name": "CastError",
    "message": "Cast to ObjectId failed for value \"{}\" (type Object) at path \"query_id\" for model \"Response\""
}

So according to the message, the field query_id is still of type ObjectId and it doesn't behave like a regular javascript object, since it needs to be cast but can't anyways.

If my observation is correct, how can I get an ObjectId field to behave like a normal object after being populated?

It will solve this problem as well:

`find` not working with nested documents after populate