Q:

React native, Whats com.facebook.react.devsupport.DevSettingsActivity

There is 2 activities inside AndroidManifest.xml when you init new project by react-native, the first one is MainActivity and the second one is  com.facebook.react.devsupport.DevSettingsActivity. What's the usage of the second one? 

A:

Adding to the accepted answer, for those who are wondering how to add this activity to debug manifest, follow these steps

Add a folder inside <project_root>/andorid/app/src and name it debug
Add a file inside and name it AndroidManifest.xml
Paste the following code in that file and save.

<manifest
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools">
    <uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW"/>
    <application android:usesCleartextTraffic="true" tools:targetApi="28" tools:ignore="GoogleAppIndexingWarning" >
        <!-- This activity will be removed for release builds -->
        <activity android:name="com.facebook.react.devsupport.DevSettingsActivity"
              android:exported="false"/>
    </application>
</manifest>

This should be done if you plan on debugging your app over the local network using wireless debugging on Android. 
Hope someone finds this helpful.

A:

You can find explanation of this activity at the header of the file 

DevSettingsActivity.java under
  $ProjectRoot\node_modules\react-native\ReactAndroid\src\main\java\com\facebook\react\devsupport\DevSettingsActivity.java:

Activity that display developers settings. Should be added to the debug manifest of the app. 
Can be triggered through the developers option menu displayed by {@link DevSupportManager}.

