xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
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        qtmfld
        
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        CuriousOne
        
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                    2Possible duplicate of [What is XML property xmlns:app?](https://stackoverflow.com/questions/38493417/what-is-xml-property-xmlnsapp) – Edric May 25 '19 at 05:44
 
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            When you want to import attributes in AndroidManifest.xml you have to kind of import their libraries. which you can then access by their prefixs {android:,tools:,app:}
Example of of usage are
android:name="yourpackege.App"
tools:overrideLibrary="yourpackege.App"
app:showAsAction="never"
In the above examples attributes being
android:name, tools:overrideLibrary, app:showAsAction
Of course every single prefix has many more attributes for example
android:name,android:theme,android:value
Kindly note though that
The app namespace is not specific to a library, but it is used for all attributes defined in your app readhere
Just to test the explanation above if you remove lets say
xmlns:android="http://schemas.android.com/apk/res/android" and rebuild your android application, you will get and error that looks something like 
        joash
        
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