C# can be used to create almost anything but is particularly strong at building Windows desktop applications and games. C# can also be used to develop web applications and has become increasingly popular for mobile development too.
Java, beyond being an object-oriented programming language with a C-like syntax, is the core execution environment within Android itself that runs many of the applications and user-land processes in the mobile OS.
Objective-C (ObjC) is a programming language that is used in the OS X and iOS operating systems and their application programming interfaces (APIs). Objective-C is object oriented, general purpose and adds to new language features in the C programming language.
Swift is a powerful and intuitive programming language for macOS, iOS, watchOS and tvOS. We can build apps for entire Apple eco-system starting from iPhone to Apple Watch and Apple TV. Swift is faster than Objective-C, because it removed the limitations of C language.
Xamarin is an open-source platform for building modern and performant applications for iOS, Android, and Windows with .NET. Xamarin is one of the most cost- and time-efficient tools used for building apps for different operating systems.