Drupal's success is largely down the number and variety of modules created by developers worldwide which add functionality to an existing Drupal website. Chances are, if you've thought of some functionality, then a Drupal developer has too, and there will be a module or modules to meet these requirements.
Wherever possible, we try to use existing Drupal modules to achieve functionality for many reasons including speed of deployment time and community support, but often functionality is so bespoke that custom module development is required.
Thankfully Drupal and many of its essential modules have open and powerful APIs which we can use to develop complex functionality with ease. We've developed hundreds of bespoke Drupal modules for different clients over the years.
Some of our Drupal modules have been submitted back to drupal.org allowing thousands of developers globally to utilise functionality we have created.
All code we develop is well-formed, commented, documented, and adheres to Drupal's coding standards.