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

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.

Generally, defining a namespace in XML doesn't has to be a real existing URL but just a world wide unique String (therefore one prefers using their own URLs). Of course, it's nice if this URL contains the XML-schema (or worse, DTD). It would be also very nice if someone would create Android Ressource Schemata. I could help him as a bachelor thesis in CS. - Prof. Solymosi, Berlin

The XML schema doesn't seem to be documented, but there is a useful list of all the layout objects and their permitted attributes here:

http://developer.android.com/reference/android/R.styleable.html#lfields

Been searching around the same subject to find out how android studio does the autocomplete & stuff in XML, and I too was hopeful to find some XSD or something, but:

In Android we use a mixture of static and dynamic DOM definitions:

1-Some files are defined using classes and annotations, see for example Manifest (note: to get correct information you should most likely use the merged manifest, but that's outside of the scope of this doc).

2-Other information is read from resources, using naming conventions to find a styleable that contains attrs relevant to a given XML tag. For example if we recognize a tag as corresponding to a View subclass in a layout file (e.g. “TextView”), we find the corresponding styleable, look at the attrs it contains and register DOM extensions for the given tag that correspond to these attr resources. See AttributeProcessingUtil and SubtagsProcessingUtil for code that reads styleables and AndroidDomExtender for the extension that plugs into the DOM system.

3-Sometimes the styleable is determined statically, but the attrs are read dynamically to stay up to date with the platform version used in the project. This is done using the @Styleable annotation.

https://android.googlesource.com/platform/tools/adt/idea/+/refs/heads/mirror-goog-studio-master-dev/android/src/org/jetbrains/android/dom/README.md

For example this is how a shape drawable XML is defined in source codes of android studio:

@DefinesXml
@Styleable("GradientDrawable")
public interface Shape extends DrawableDomElement {
  @Styleable("DrawableCorners")
  List<DrawableDomElement> getCornerses();

  @Styleable("GradientDrawableGradient")
  List<DrawableDomElement> getGradients();

  @Styleable("GradientDrawablePadding")
  List<DrawableDomElement> getPaddings();

  @Styleable("GradientDrawableSize")
  List<DrawableDomElement> getSizes();

  @Styleable("GradientDrawableSolid")
  List<DrawableDomElement> getSolids();

  @Styleable("GradientDrawableStroke")
  List<DrawableDomElement> getStrokes();
}

The styleables (like GradientDrawablePadding) are defined in android's attrs.xml

https://android.googlesource.com/platform/tools/adt/idea/+/refs/heads/mirror-goog-studio-master-dev/android/src/org/jetbrains/android/dom/drawable/Shape.java