Where are the schemas for XML files on an Android project?

Where are the schemas (DTD or XML schema) for the XML files used on Android like AndroidManifest.xml or the layouts?

From stackoverflow

• The schemas don't exist as an xml file. Schemas are dependent upon what UI classes your program uses. There's a slightly better discussion here.

  Casebash : What does it mean to write: `xmlns:app="http://schemas.android.com/apk/res/com.example.android.apis"` then?