I am doing my bathroom over. There is still some old pipe in the walls that looks to be 3/4" threaded brass. In some places the 3/4" brass has been connected to copper. It looks like the 1/2" copper was just inserted into the cut 3/4" brass and then soldered.

I have to do something similar and I am wondering if this is the correct approach. If yes, is there any trick to doing it? If no, what is the correct way?