And by "core," I mean the essence that makes your project, job, or hobby what it is.

我所说的“核心”，是指你的项目、工作或者兴趣还好的本质。

youdao

Two members of the group, who identified themselves only as "John" and "Liu, " told CNN in a phone interview that a core group of six people on three continents worked to unlock the iPhone as a hobby.

CNN: Code to unlock iPhone cracked