Cocoa, which is a high-level framework targeted at GUI app development, does require either Objective-C/C++ or Swift. (There are third-party bindings to other languages, like Xamarin/C##.)
There are a number of C-based high-level frameworks, such as Core Foundation, Core Graphics (a.k.a. Quartz), etc.
The high-level frameworks are documented online and in Xcode's documentation viewer. (Although I feel that Apple's documentation has been getting steadily worse over the years.) Sometimes, the nitty-gritty details are only documented in the respective header files.
For system programming, macOS is Unix. There are the standard C library, the standard C++ library, and the POSIX/BSD library and syscalls. The headers in …/usr/include are and will remain supported, in general; specific (old) functions may be marked as deprecated. The compiler should warn you about those. The POSIX/BSD APIs are documented in the man pages installed with the developer tools. Just "man <function>" at the command line will get you the up-to-date docs. I don't know if that counts as "comfortable, modern and actual".
Older documentation can become a bit out of date, but is still usually fundamentally correct.
If you want more specifics, you'll have to explain specifically what you're trying to achieve and can't figure out how to do.
Yes, macOS is Unix, and POSIX-compatible, but unfortunately is not fully.
Some system calls are missed in macOS or exist, but makes SEGV.
For example, semaphores - in documentation we could find system call sem_open (sem_init doesn't exist), but we get SEGV (or it just will not work - I don't remember) if we will try to use it in our program. Why? Because we should use
dispatch_semaphore_create. But see again manpage for sem_open - nothing is said there about it. Thanks God we have StackOverflow. Official documentation for
dispatch_semaphore_create is only for Obj-C/Swift, but fortunately this syscall is also available for C.
When I wrote about comfortable documentation with search, I meant something like this. But it is not official documentation. As I understand something similar, but official doesn't exist =( Okaay. Just macOS is not Open-Source project and therefore I think this situation is a little bit strange.
I just searched any "official documentation", but as I understand, official documentation is manpages =) Okey, now I know
And also I'm confused in Apple API's. Now it is more clear