.addin manifest
Short answer
The XML file Revit reads at startup to discover and load .NET add-ins from the user's Addins folder.
A Revit add-in is registered via a small .addin XML manifest dropped in %AppData%\Autodesk\Revit\Addins\<version>\. The manifest names the DLL, the entry class implementing IExternalApplication or IExternalCommand, and a stable AddInId GUID. Revit scans the folder at startup and loads every manifest it finds. Installer packages typically drop both the .addin and the DLL into this folder.