These are the steps required to build an Android application out of Unity and put it onto a mobile device. You can build to an Android Device from both Mac and Windows.
Note: Both iOS and Android require a Bundle ID (called a Package
1: Download the Java Development Kit (Version 8 - make sure it is version 8) from here: Java Development Kit (JDK) and install.
2. Install Android Studio by downloading it from here: Android developer portal. Follow the instructions from Google for it's installation.
3. Open Android Studio and go to the SDK Manager.
4. From the Android SDK platforms, select and install the platform you will be building for. In my case, my phone has Android 8.0 (Oreo) installed but the settings in Unity for the minimum Android API for Wikitude are set to KitKat so I have included both.
Take note of the Android SDK Location as you will need to provide this to Unity.
On the device under Settings > About phone > Software Information you can find the Android platform information.
While you are on this screen on your device you should activate Developer Mode. To do this tap on the Build Number 7 times.
5. Under Settings on the mobile there will now be a Developer Options screen. Go into this and scroll down to turn on USB Debugging. This will give your computer (and Unity/Android SDK) access to the mobile device.
Higher up on this screen is an option to Stay awake. I turn this on to stop my devices locking up and going to sleep while I am developing.
6. Download the Android NDK version r13b (64-bit) from https://developer.android.com/ndk/downloads/ or if you require an older version because Unity insists on it visit https://developer.android.com/ndk/downloads/older_releases. Extract the android-ndk folder to a directory on your computer and note the location.
7. Create a new Unity Project. It can be empty with just the Sample Scene.
8. In Unity from the Main Menu select Unity > Preferences (Mac) or Edit > Preferences (Windows). In the Android section add the directories containing the Android SDK, JDK and NDK that you've just installed.
9. From the Main Menu select File > Build Settings (a). This will open a Build Settings Dialog box where you should ensure the scene/s that you want to build into your app are listed in the top box. If they are not you can drag them from the Project window into here (b). Then select Android in the Platform box (c). Then finally click on "Switch Platform" (d).
Switching the platform will be relatively quick with an empty project. If you already have an application and want to switch the platform Unity will perform some additional compilation and reimports and this could take a little while depending on the size of your application.
10. The platform you are currently set to will be displayed in the top menubar of Unity.
11. Open the Build Settings window again. (Main Menu > Build Settings) This time click on the "Player Settings" button (a). In the Inspector add a Company Name (b), Product Name (c) - this is the name of the app that will appear on the mobile device below the icon and the Package Name (this will be the same as the Bundle ID if you are also building for iOS) otherwise the format is com.yourname.appname (d). Then to deploy to your mobile device click on "Build and Run".
12. With your mobile device while you do this or maybe previously when you plugged the mobile into the computer because a screen asking for USB debugging permission will pop up. Select OK when you see this. Make sure you keep an eye on your mobile while building for any messages that might need action because if Unity fails to build you might not see what the issue was.
13. After the build the application will run on your mobile device.
What if it doesn't work or I get strange messages?
Make sure your device is plugged in and USB debugging is enabled.
Ensure the Android SDK you are building to (on the device) has been downloaded and installed via the SDK Manager in Android Studio.
More Useful Information for Android App Building from Unity