Hey Erik,
Adding something into the core folder would suggest you're not using the internal events system. We've built Adapt to have a pluggable core that enables Plugins/Extensions to tap into core and retrieve/update data.
We're in the process of creating some documentation about getting to know core.
To help guide you along the way please take a look at the following documentation:
Adapt API
This is a detailed overview of the Adapt object you can require in an extension. It should give you all the handy information you require to manipulate data.
https://github.com/adaptlearning/adapt_framework/wiki/Adapt-API
Core Events
You should be using the events system in Adapt. Here you'll find all the useful information and list of events Adapt passes out.
https://github.com/adaptlearning/adapt_framework/wiki/Core-Events
Core Modules
This details all the core modules inside Adapt. This includes the Router, Drawer and Notify.
https://github.com/adaptlearning/adapt_framework/wiki/Core-modules
This should give you a starting point to creating an Extension. I'd suggest looking at something like adapt-contrib-resources for the structure and how to setup the events.
In terms of developing in Adapt, Extensions are the most complex to write - so if you're just starting out, we suggest writing a component first to get use to the way Adapt works.
Thanks and hope this helps,
Daryl