db.t1.updateOne({_id: 1, y: 3, x: 2}, {$set: {b: 2, d: 4}, $inc: {a: 1}, $push: {e: 5}, $setOnInsert: {c: 3}}, {upsert: true});
// result
[
  {
    // filter fields are added as it was passed in the command without any ordering
    _id: 1,
    x: 2,
    y: 3,
    // fields mentioned in update operators are sorted and processed one by one.
    a: 1,
    b: 2,
    c: 3,
    d: 4,
    e: [ 5 ]
  }
]

db.t1.updateOne({_id: 2}, {$set: {"a.y": 2}, $inc: {"a.x": 1}, $setOnInsert: {"a.z": 3}}, {upsert: true});
// result
[
  {
    _id: 2,
    // fields with dot notations are sorted and processed (similar to previous case)
    a: { x: 1, y: 2, z: 3 }
  }
]

db.t1.updateOne({_id: 3}, {$set: {x: { c: 3, b: 2}}, $setOnInsert: {a: 1}}, {upsert: true});
// result
[
  {
    _id: 3,
    // sorting is only done for fields at root level
    // if the operator has as a field whose value is a document (in our case of `x`), then the value is treated as it is, sorting is not carried out with fields inside that document (value).
    a: 1,
    x: { c: 3, b: 2 }
  }
]

db.t1.updateOne({_id: 3}, {$set: {"x.b": 4, "x.c": 5, "x.d": 6, "x.a": 3}});
// result
[
  {
    _id: 3,
    a: 1,
    x: {
      // existing nested fields x.c and x.b retained their position
      c: 5,
      b: 4,
      // x.a and x.d are sorted and appended
      a: 3,
      d: 6
    }
  }
]