composer installcommand from inside "themag-thunder-project" directory. Composer installs Drupal itself in the "docroot" directory (themag-thunder-project/docroot). Consider that on a production environment you have to configure your domain to point in that directory.
cd themag-thunder-project composer install
vendor/bin/drush site:install thunder \ --existing-config \ --db-url=mysql://db_user:db_password@localhost/db_name \ --account-name="demo" \ --account-pass="demo" \ --account-mail="firstname.lastname@example.org" \ thunder_module_configure_form.install_modules_thunder_demo=NULL
vendor/bin/drush en -y themag_thunder_demo
The following articles will help you to understand how to manage your Drupal site with Composer:
Using Composer: https://www.drupal.org/docs/develop/using-composer
Drupal 8 Composer Best Practices: https://www.lullabot.com/articles/drupal-8-composer-best-practices