Where is Twig service defined?
The class that registers the Twig service is CoreServiceProvider which, in CoreServiceProvider::register() (the method that every service provider needs to implement), uses the following code.

  // The 'request' scope and service enable services to depend on the Request
  // object and get reconstructed when the request object changes (e.g.,
  // during a subrequest).
  $container-&gt;addScope(new Scope('request'));
  $this-&gt;registerTwig($container);
  $this-&gt;registerModuleHandler($container);


The class used for the Twig service is not the Twig_Environment class that Symfony implements, but a subclass of it (TwigEnvironment) implemented by Drupal.

The other registered service is twig.loader.filesystem, which uses the Symfony's Twig_Loader_Filesystem class.