Ntdll being the de facto syscall abi is the right answer. But ntdll is also technically subject to change, much of it not publicly documented by Microsoft. So typically a third party application calls into things like kernel32 or advapi (though Win7 and later split those into different DLLs internally) which wraps ntdll which does the actual ordinal calls.

But generally speaking, yeah, the stable interfaces are the C ones, exposed through system DLLs. Linux is unique in developing libc and kernel separately and the division between the two as a public abi boundary.