Zenario's functionality is provided by its Modules.
Core modules are found in the zenario/modules directory - don't modify these (if you modify them, when you upgrade Zenario you would need to re-apply your edits).
Put your custom modules in zenario_custom/modules - this directory is within zenario_custom and so does not get overwritten in core Zenario version updates.
When you write custom modules, they can add to, override or even turn off functionality created by Zenario's core modules; so there should never be a need to modify the core.
Modules are the way of adding functionality to Zenario. This page covers fundamentals of modules and their naming conventions.
Modules will not start running immediately when you put them in the zenario_custom/modules/ directory, you need to start them first.