What is the best way to load 2 million records from DataTable to CSV file in asp.net core c#?

Question

I am exporting a CSV file with 2 million records and 150+ columns. It is utilizing high RAM around 30-40 GB. What is the best way to load this data into a CSV file?

using (Stream stream = File.OpenWrite(fileInfo))
                    {
                      stream.SetLength(0);
                      using (StreamWriter writer = new StreamWriter(stream))
                        {
                          writer.WriteLine(string.Join(Seperator, newColumnList.ToArray()));
                             foreach (DataRow row in dataTable.Rows)
                               {
                                IEnumerable<string> fields = null;
                                if (fileextension.ToLower() == "txt")
                                  {
                                    fields = row.ItemArray
                                   .Select(field =>
                                           field.ToString());
                                  }
                                 else
                                  {
                                   fields = row.ItemArray.Select(field =>
                                    string.Concat("\"", field.ToString().Replace("\"", "\"\""), "\""));
                                  }

                                writer.WriteLine(String.Join(Seperator, fields));
                             }
                            writer.Flush();
                          }
                      }