Respectively, the Core, the Lifespan and Natural recovery, the Divine Senses, the Life Bound Item, and the Incomplete domain called the Unbreachable Shield!
The Core of/on the cultivator's body after advancement toward the Core Formation realm could be said the advanced utilization of Qi!
After reaching the peak of the Foundation realm, the Qi inside the cultivator's body and heart couldn't advance anymore due to them filling the cultivator's body with Qi...
So in order to advance, the cultivator needed to create a small sphere made of pure and purified Qi in certain parts of their body (depending on the cultivation technique they practiced), making it the core of the cultivator.
At the start, the Core of the early stage of the Core Formation realm only took a gaseous form, followed by the liquefied in the middle stage, solid in the late, and tempering the Core at the peak stage!