Declare your new activity in your manifest.xml

To declare a new activity in your Android app's manifest.xml file, you need to add a <activity> element inside the <application> element. Here's an example:

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.myapp">

    <application
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name">

        <!-- Declare the new activity -->
        <activity
            android:name=".MyNewActivity"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <!-- Other activities, services, and receivers... -->

    </application>

</manifest>

Let's break down the elements:

• android:name: specifies the fully qualified name of the activity class, e.g., com.example.myapp.MyNewActivity.
• android:exported: specifies whether the activity can be launched from outside the app. Set to true if you want the activity to be accessible from other apps or from the device's home screen.
• intent-filter: specifies the intent filters that the activity responds to. In this example, we're using the MAIN action and LAUNCHER category to make the activity the default launcher activity.

Note that you need to create a new Java class that extends the Activity class and matches the name specified in the android:name attribute. For example:

public class MyNewActivity extends AppCompatActivity {
    // Activity code goes here
}

Make sure to update the AndroidManifest.xml file and the Java class accordingly, and then rebuild and run your app to test the new activity.