Usually, each folder in the root of the website is core (but not necessarily).
Core — is a collection of modules, templates, and virtual root folder.
The framework is the core, your folder will also be the core (you need to create it), each core should be stored in separate repositories.

In the simple case, you will have only two cores, for example: /core for the framework and /site for your project.
It is possible to add additional cores.
All active cores need to be registered in the /config.php file (the 'cores' key).

When the framework class Engine looks for a file to include, it goes through all cores registered in the config from the first key to the last. Once the desired file is found, Engine connects it, skipping the further search. In this way, you can redefine any framework file by creating a file in your core in a similar relative path.

The following folders may be located inside the core:

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