Most companies e.g. Unreal charge a fee for using their SDKs.
The idea that you would be forced to give it away for nothing would be a pretty extraordinary and unworkable intervention in the market.
The idea that you would be forced to give it away for nothing would be a pretty extraordinary and unworkable intervention in the market.
So at some point you have to interact with Apple SDKs.
In almost every other case application software is not considered to be a derivative work of the operating system it runs on. If this wasn't the case, then Apple could have easily sued Cydia and AltStore for offering the equivalent of iOS fanfiction. Hell, even the Copyright Office was perfectly fine with adding a DMCA 1201 exemption for jailbreaking iPhones to install non-Apple software on them - and they're extremely tightfisted with those.
The reasons why this is different is very simple: you don't distribute Apple's SDK along with your application, but you do distribute Unreal's. The user got access to Apple's code when they bought their iPhone, you have to give them Unreal Engine, so you need a license for that.
1 million downloads for an app sounds like a lot for a paid app.
But on the desktop, popular OSS software packages do those kind of numbers in well under a year. There's no reason to believe OSS mobile apps on iPhone would be any different.