Think of the ExtensionsIndex as a repository containing a list of extension description files (*.s4ext) used by the Slicer extensions build system to build, test, package and upload extensions on an extensions server.
Once uploaded on an extensions server, within Slicer, extensions can be installed using the extensions manager.
A website referred to as the extensions catalog provides Slicer users with a convenient way of finding and downloading extensions previously uploaded on the extensions server. The extensions manager in Slicer makes the catalog available directly in the application and allows extensions to be installed, updated, or uninstalled by a few clicks.
The following diagram depicts how extensions catalog, extensions server, CDash and the
slicer factory interact.
See https://slicer.readthedocs.io/en/latest/developer_guide/extensions.html#extensions-build-system
Questions regarding this project should be posted on 3D Slicer forum: https://discourse.slicer.org