manager-bundle/src/EventListener/InitializeApplicationListener.php
public function onInitializeApplication(): void
    {
        if (file_exists($this->projectDir.'/system/initialize.php')) {
            return;
        }

        (new Filesystem())
            ->copy(
                __DIR__.'/../Resources/skeleton/system/initialize.php',
                $this->projectDir.'/system/initialize.php',
                true
            )
        ;
    }
public function onInitializeApplication(): void
    {
        if (file_exists($this->projectDir.'/system/initialize.php')) {
            return;
        }

        (new Filesystem())
            ->copy(
                __DIR__.'/../Resources/skeleton/system/initialize.php',
                $this->projectDir.'/system/initialize.php',
                true
            )
        ;
    }
Adds the initialize.php file.