Before creating a Java or native C plugin for an Android platform inside Unity, you should install the Android NDK. Also, if you don't know how to build a shared library, then you should find out more about this process. A lot of information about Android NDK can be found on the web, for example, the Android official documentation can be found at https://developer.android.com/tools/sdk/ndk/index.html or in many different books by Packt Publishing; for example, see visit https://www.packtpub.com/application-development/android-ndk-beginner's-guide. Information about Android NDK is beyond the scope of this book.

Some parts of your game or app can be implemented using native-code languages, such as C or C++. The Android NDK is a toolset with multiple features and possibilities. You will not need to use the Android NDK in every project, but in some games or applications, it will be very helpful to reuse some custom or third-party code...