ECA stands for "Events - Conditions - Actions" and is a powerful, versatile, and user-friendly rules engine for Drupal 9+. The core module is a processor that validates and executes event-condition-action plugins. Integrated with graphical user interfaces like BPMN.iO, Camunda or other possible future modellers, ECA is a robust system for building conditionally triggered action sets.
Drupal
The enterprise OpenSource content management system which is available for free at https://www.drupal.org where you can find all the details and also references.
Drupal has a very active community with tens of thousands of developers all around the world and they also have a huge focus on standard compliance and on security.