Apple's gross revenue was about $14 billion, not $50 billion. Total app store revenue in 2019 was $61 Billion. Apple received between 15% and 30% of that, depending upon the first year/later years subscription numbers. If we use 22.5%, Apple made $13.7 billion.
https://www.theverge.com/2020/6/15/21292203/apple-app-store-...
Follow the link to the cited study. Another data point is Apple's release that they paid developers $34 billion in 2018.
https://fortune.com/2019/01/28/apple-app-store-developer-ear...
Since the 15% subscription model was relatively newer in 2018, let's put the 2018 developer number at 75% of the total, so Apple made about $11.3 billion in 2018.
Also, we should not forget the huge overall R&D spend Apple is making to keep growing the iPhone/iPad market. Apple makes money off sales of course, but it is still a huge investment in the platform and that creates market for developers.
They have also just announced a new platform for Macs that will allow iPhone and iPad apps to run unaltered on the new Macs. Again, big R&D expense from Apple with upside for developers. In general, there have been big R&D expenditures in software SDKs in recent years for developers to get to a write once model so programs can run on all the platforms.
Apple also eats the 2.5% (typical number) credit card transaction costs. In addition, they deal with collecting and paying local sales taxes in every state and in 155 countries.
For the user, they provide a trusted payment processor and software source so they don't have to worry if the software will have embedded malware or if their credit card data will be handled appropriately.
They provide cloud servers for notifications, file storage, all of the software update processing, payment processing, and many other services.
The app store team performs about 100,000 reviews every week. Somewhat automated, but a lot of humans involved too.
After the first year, Apple's cut is 15%, so after all the other expenses of the app store I'd be surprised if they are clearing 10% after that. Whatever the exact number, it is far less than 30%.
The 2019 net prior to R&D and other expenses is probably around $12 to $13 billion best case. Developers made about $47 billion. At any rate, Apple is making money, but we should keep the right numbers in mind when we have these discussions.