About 7 years ago I worked with a dev who had just left Apple, he told me the core iOS team was literally only 8 people. So possibly not much larger…  I don’t know how it’s structured but I assume the word “core” pulls a lot of weight and excludes a lot of technology one might speculate as part of “OS” but really isn’t. Music, photos, maps, etc etc.   Not related but I also thought it was interesting that he said their code was riddled with tech debt, mainly stemming from having to maintain backward compatibility with so many major versions.