Develop Reference

Where does a native *.so library and its headers go in an android project?

I have looked at Use prebuilt JNI library in Android Studio 3.1 and How to use .so in a second project in Android?. The first is trying to get a library file without headers working and the other seems to be focusing on a specific issue with his build (although there's some useful information there). I'm relatively new to app development and especially to native development on android. I've gotten a build with the JNI library and some c++ code working, but that seems to be just for building from source.
It's probably a simple answer, but I haven't been able to find documentation on this specifically in the android developers documentation. I'm interested in understanding the correct (or most conventional) place to put and way to use a precompiled library (module/lib/*.so and module/include/*.h) in an android project. Would I even need to use JNI or the NDK if the library is built with another build tool? Another project I have has a native library source object (*.so) in ./obj/local, ./libs, and in many other folders related to JNI. I'm guessing it would be somewhere in there, but I'd like to know what is conventional.
For some context, I'm trying to work with the essentia library. I have followed the guide on compiling for Android and have a build with the general hierarchy mentioned above (essentia/lib and essentia/include) that seems to be working.

Writing apps in C++ in Android Studio?

I was wondering if writing apps in C++ in Android Studio is possible. I have some knowledge in creating apps in Java but I am more comfortable with C++, and is this a complicated process?

You can use Qt and create Android apps with C++ (http://doc.qt.io/qt-5/androidgs.html).
To setup Android Studio with Qt https://wiki.qt.io/Android

You can add C and C++ code to your Android project by placing the code into a cpp directory in your project module.
When you build your project, this code is compiled into a native library that Gradle can package with your APK.
Your Java or Kotlin code can then call functions in your native library through the Java Native Interface (JNI). If you want to learn more about using the JNI framework, read JNI tips for Android.

You can also use JNI to run c++ code (as a matter of fact c++ shared library or so) from Java.
Here you can find a usefull article + sample for how to do it.

Integration of Android Native module with existing Xamarin Native Project

We have teams which are working on different different technologies. Currently struggling with integration of android native module (Developed in android native language) with existing Xamarin android project.
Actually I got one way that we can convert android native project as a library. This library can integrate to Xamarin project.
I had gone through below links
Convert existing project to library project in Android Studio
https://developer.xamarin.com/guides/android/advanced_topics/using_native_libraries/ ( Integration of android library in xamarin project)
Is this the correct way or do we have any alternate way to achieve it?

Is this the correct way or do we have any alternate way to achieve it?
The document Using Native Libraries is talking about how to use .so file (native libraries) in Xamarin.Android, here is the example.
It is recommended that using Binding Java Library. You could convert your android native project to a .jar or .arr file, then you could refer to Binding a .JAR or Binding an .AAR.
If there is any problem when you trying binding Java Library, you could refer to Troubleshooting Bindings.

Android Studio - Add shared library (.so) file and header (.h) file to project

I am currently working on an android project that requires me to make use of functions included in a shared library (.so). I also only have header (.h) files for the library provided to me.
Is it possible to work with just these two files? Or do I need to create my own implemenations via c++ codes?
I am using Android Studio intend to use CMake.
Regards,
Philip

Most Android apps are written in Java. Google has released the Native Developer Kit (NDK) in order to allow developers to write libraries in C++. However, these libraries are usually very low level and called from the Java code which defines the UI and higher-level app logic. Most likely you will need to write a wrapper for the library so that you can call it from Java code. Looks like this blog is a good place to start.

How to call methods from a C++ library in Android?

I have a huge C++ library for image processing. I would like to use this DLL library in an Android project. How can I call methods in the C++ DLL library in Android?

Maybe you can try JavaCPP. JavaCPP will help you auto generate appropriate code for JNI, and passes it to the C++ compiler to build a native library.
For more details and examples, please see： https://code.google.com/p/javacpp/

via JNI Java Native Interface.
using this keyword will let you find a tutorial:
http://developer.android.com/training/articles/perf-jni.html

You need the Java Native Interface (JNI)
Oracle has a great set of tutorials on it (google for them).
Also, since you tagged your question Android, I'm presuming that's the platform you need JNI on.
You'll have to look into the Native Development Kit, more info on that here: http://developer.android.com/tools/sdk/ndk/index.html