The source of the asset is inside: Assets/hardartcore/In-App Updates folder. You can find everything you needed there.
Inside Demo folder you will find a simple demo scene with a basic implementation with subscribing for all the events from UpdateManager class and showing information about the download process when there is an update available
Inside Plugins folder you will find the native Android Java .aar library with a txt file with proguard rules which you should include in your proguard file if that option is enabled for you.
Inside Prefabs folder you will find two prefab files: UpdateManager & AutoUpdateManager. The difference between both prefabs is that AutoUpdateManager will take care for everything, you just need to drag & drop it in your Scene and you are good to go. However if you need to customize the process of informing the user about the update, show progress of bytes downloaded vs total and etc. you should use UpdateManager which will let you customize the user experience. You can check the DemoScene inside Demo folder to see a sample implementation of that.
Inside Scripts folder you will find both AndroidUpdateManager.cs & AutoUpdater.cs. That's where the magic happens!