In my project there is an app and a mod, application and android library respectively.

I am learning about android studio Gradle by following an online tutorial. In the tutorial I see androidApplication = “8.1.3”. When I checked all versions of Android Plugin versions, I do not see 8.1.3. I see 8.1. Also I see using BaseExtension class when creating an extension class. Is this class a part of the version 8.1.3. I tried to find out about this class but could not find. Please suggest me the right direction to read the Android Developers documentation. Thank you.