You are correct Apple Music has low margins (Less than 30%), but it also only has 40M subscribers, which is likely less than $5B in annualized revenues, or roughly 10% of total services revenues.
So $50B in services at 68% margins equals $16B in expenses, deduct $4B in Apple Music expense and you have $12B left. Deduct $9B from revenues for Googles search engine placement, $5B more in revenues from Apple Music, and you have $36B in revenues left. That makes gross margins for the remaining services, which the App Store is 40%, 67%.
You are correct in that 67% is closer to 80% than 50%, but still a long way from it.
I've not done the research, but following your #s, there remains 60% of the $36b unaccounted for. Where's that coming from, and what are those margins?
There's zero percent chance the App Store is running at 50% margin. It's got to be closer to 80% than 50%, or they are doing something wrong.