Question

How to write ‘Hello World’ string, clrf, and some random 10 bytes to a memory stream in Delphi?

Answer 1

I would consider using a binary writer for this task. This is a higher level class that takes care of the details of getting data into the stream.

var
  Stream: TMemoryStream;
  Writer: TBinaryWriter;
  Bytes: TBytes;
....
Stream := TMemoryStream.Create;
try
  Writer := TBinaryWriter.Create(Stream);
  try
    Writer.Write(TEncoding.UTF8.GetBytes('Hello World'+sLineBreak));
    //if you prefer, use a different encoding for your text
    Bytes := GetRandomBytes(10);//I assume you can write this
    Writer.Write(Bytes);
  finally
    Writer.Free;
  end;
finally
  Stream.Free;
end;

I expect that your real problem is more involved than this. The benefit of using the writer class is that you insulate yourself from the gory details of spewing data to the stream.