Activity was originally the only application entry point that Unity supported and because of this it’s very stable in the majority of scenarios and is compatible with the majority of Unity features.
If you use Activity, your application player loop runs on a Java thread. This means that you can call Java APIs like myLooper from plug-insA set of code created outside of Unity that creates functionality in Unity. There are two kinds of plug-ins you can use in Unity: Managed plug-ins (managed .NET assemblies created with tools like Visual Studio) and Native plug-ins (platform-specific native code libraries). More info
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If you use Activity, Unity uses the Java choreographer. This helps you to understand how frame synchronization occurs in your application.