u/Right_Inevitable_864

I have been trying to understand how the most mammals are not descended from monotremes. From what I have looked at it seems as though the Theriiformes diverged from creatures that operated essentially the same as the monotremes did. Am I misunderstanding something or is it just that the continuation of traits in one group as well as a divergence in the other meant that the old group must be recategorized after that split point. It may be a dumb question I just would like to know for certain as I am not sure I am understanding correctly.