Autoload Mapping for modules
Drupal 8 follows PSR 4 autoloading standard which is implemented by composer autoloader.

While some Drupal namespaces are registered statically in composer.json and core/composer.json other namespaces are processed dynamically (see DrupalKernel::classLoaderAddMultiplePsr4).

PSR-4 namespaces and autoloading in Drupal 8