composer installcommand from inside "themag-drupal-project" directory.
cd themag-drupal-project composer install
Note that drupal-project installs Drupal itself in the "web" directory (themag-drupal-project/web). Consider that on a production environment you have to configure your domain to point in that directory.
cd web drush site:install \ --existing-config \ --db-url=mysql://db_user:db_pass@localhost/db_name \ --account-name="demo" \ --account-pass="demo" \ --account-mail="email@example.com"
Change db_user, db_pass, and db_name with your database info. The --account-name is your Drupal username and the --account-pass your Drupal password.
drush en -y themag_demo
This articles will help you to understand how to use Composer to manage your Drupal site:
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