If you’re a developer and looking into contributing to ARK Core then this is the right starting point for you to get familiar with how it is structured.
packages directory contains all of the official packages that are developed and maintained by the ARK Ecosystem development team. You should never add your own packages to this directory.
plugins directory contains all of the packages that you might be working on locally. You should always add your own packages to this directory.
__tests__ directory contains all of the tests that you might be working with when preparing a pull requests. You should never add tests for your own plugins to this directory.