> How do you not use their SDK?

The first third party iPhone apps existed before the SDK existed. iPhone OS was jailbroken pretty quickly and apps emerged fast. SDKs are nice references and tools but the're not the only way to skin a cat.

Nowadays with LLMs people could probably patch up some FOSS SDK that perfectly replicates the original's functionality without infringing Apple's copyrights. If the DMA was enforced correctly this would have already happened.

Personnally I use Flutter which has a very minimal use of their SDK.

Basically enough to render inputs to a gpu accelerated surface.

And I'd argue that this is a core OS feature that you expect on any modern OS.