How and where does the Facade class property $app get set?

I have done a lot of digging and cannot seem to see where or how the $app property is set in Illuminate\Support\Facades\Facade

I am aware all Facades are set in config/app.php and auto loaded using the Illuminate\Foundation\AliasLoader however i can see the auto load and class_alias, I however cannot see where the main $app is passed or injected?

via Chebli Mohamed