If you are unfamiliar with how to setup an Android application, I strongly recommend you take a look at our Android examples. It isn’t really in the scope of our forums to show you how to setup an Android application- for that you should refer to the Application fundamentals section of the official Android documentation.
I know it is not very intuitive and there is a lot of information out there, so I will try to point you in the right direction.
The Qt Android examples provide a good template for how to setup a Qt Android project. In their examples, activities are placed in “Other files/android/src/org/qtproject/example/activityhandler”
You can call your activity whatever you like, and there are many ways to run it, but it does not run by default. The simplest thing to do would be to declare it as your main action in your manifest.
<application android:label="My App" android:extractNativeLibs="true">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>