Question: 1. Are there any guarantees that C++ std IO will be portable across all Desktop and Mobile OS (I'm interested in iOS and Android)?
2. Is implementation of std IO different across the platforms or it is rather uniform? If it is different, then does it happen due to SDK of the platform (in other words - do SDK's provide those different implementations)?
3. Who provide those implementation? Who is the author? Does anybody update them?
4. Where is documentation?

Answer: 
> 
> Are there any guarantees that C++ std
>  IO will be portable across all Desktop
>  and Mobile OS (I'm interested in iOS
>  and Android)?
> 
> 
> 

No, there are no guarantees that these platforms will implement, correctly at all the standard library. 

> 
> Is implementation of std IO different
>  across the platforms or it is rather
>  uniform? If it is different, then does
>  it happen due to SDK of the platform
>  (in other words - do SDK's provide
>  those different implementations)?
> 
> 
> 

It's different. I/O is very different on different platforms.

> 
> Who provide those implementation? Who
>  is the author? Does anybody update
>  them? Where is documentation?
> 
> 
> 

Either the compiler implementor or the platform owner provides them. The C++ Standard describes what the library must do.
The idea of a standard (hence the std) is that it is respected and uniform no matter what platform you are on.
Some developers ship devices with support for all or some of the std library, it's really just up to them how it is implemented.

This is platform specific and probably available in each platform's SDK documentation, available most probably with the SDK or on the vendor's website.