Actually it was almost the opposite. In this case I think you are remembering the events of one release where Apple had to delay an iOS release and pull more re

Actually it was almost the opposite. In this case I think you are remembering the events of one release where Apple had to delay an iOS release and pull more resources to get it thought. A couple minutes of searching has not found it for me. But in general the motion has been in the opposite direction. In the beginning iOS was its own project, and had a dedicated staff that took from MacOS (both staff and code) and did their own things with them. Once iOS was successful they decided to merge the parts that made sense to merge, and did so back into the MacOS teams. So there are a lot of teams that are shared between MacOS and iOS projects, and a long-term goal of merging as much of the codebase into one as makes sense (to Apple). An example of this is APFS, which was partially written to resolve the differences that had grown between the MacOS and iOS versions of HFS+.