Xamarin provides a number of ways to save data in iOS using Local Storage. User preferences are generally stored in Library/Preferences/.
Xamarin iOs provides its built in mechanism i.e. instead of creating files directly in the directory it is suggested to use NSUserDefaults class to save data.
NSUserDefaults class provides its own built in mechanisms to save float, double, integers, Boolean and URL data. To save a custom object it should be converted to NSData object using NSUserDefaults.