The Elbow.ByMepCurves node looks for the closest connector for each connector and then creates an elbow. If you want Tees or Elbows or Unions then you will need to use these nodes:
The difference with Elbow.ByMepCurves is that you have to input the exact pipes that you want to connect to each other.
I have not made a node for creating crosses, but you could use the code from this post: