Dear Student,

I don't know what are you talking about.The first most skill you required to become Operating System developer is to learn C/C++ skill as the windows operating system drivers are written in C/C++ language.You should know the data structure and classes for making the drivers for OS

For IOS developer you should know the basics syntax of SWIFT language.You should know UI& UI for making good looking app.You should know the Mode View Controller(MVC) design pattern and the most basic thing Git& Github which every developer should know now a days without git & github your work will be very difficult to handle as a developer doesnot matter which developer you are Web/Android/IOS.I suggest you to first learn UI&UX which is in great demand in today's world then git & github then learn SWIFT language go slowly slowly but do practice daily.