Extend Drupal
Drupal like other CMS or open source platform is so extendable with modules and themes available from the community. The Drupal community I found it is amazing and very supportive.
Installing module and theme with composer
To install Drupal module or theme with composer you can just simply run
composer require drupal/module_name
Example above is for a drupal project called module_name and in Drupal page it will be after /project/ folder: https://drupal.org/project/module_name
Install module with composer require issues
Memory exceeding
Sometimes the composer as a memory limit issue, add the following command before the composer.
php -d memory_limit=-1 `which composer` require drupal/module_name
# instead of
composer require drupal/module_name