To add additional block regions in a Drupal theme you need add the new regions to the following files. In this example I am adding four blocks side by side 25% width just above the content block. As I am using a Bootstrap theme, I have wrapped them in the div col-sm-3.
theme.info.yml - you define them in this info file in this format:
As a Drupal developer, I use composer which is a dependency manager for PHP. A big part of developing Drupal 8 and above is to be able to use composer to install Drupal core, modules, themes and all dependencies without having to manually install everything individually. It is now recommended to install a Drupal module using composer to download it's dependencies.
