example.dart

void main() {
  List<Map<String, dynamic>> arr1 = [
    {"id": "1", "name": "First Item", "price": 10},
    {"id": "2", "name": "Second Item", "price": 12}
  ];

  List<Map<String, dynamic>> arr2 = [
    {"id": "1", "quantity": 1},
    {"id": "2", "quantity": 3},
    {"id": "3", "quantity": 2}
  ];

  final arr3 = arr1.map((product) {
    final quantity = arr2
        .where((quantities) => quantities["id"] == product["id"])
        .map((quantities) => quantities["quantity"] as int)
        .first;
    return product..["quantity"] = quantity;
  });

  print(arr3);
}