Comment by pjmlp

Android, userspace is Java, and what is exposed on the NDK is a tiny portion, as it is only meant for games and implementing native methods for better performance beyond what JIT/AOT do, or bindings to existing libraries.

About 80% of the OS APIs are behind JNI calls, when using the NDK.

iOS, iPadOS, watchOS, the large majority of userspace APIs is based on Objective-C, or Swift, bare bones C is only available for the POSIX leftovers.

You need to call the Objective-C runtime APIs for anything useful as an app that Apple would approve.

For the Plan 9 geeks, Inferno, OS APIs are exposed via Limbo.

For folks that still find mainframes and micros cool, IBM i, IBM z/OS, Unisys ClearPath MCP, Unisys OS 2200.

For retrogaming folks, most 8 and 16 bit home computers.