To put it simply, cultivation in the Primordial Sea Realm is about accumulating True Essence, filling the Dantian, and Transforming Qi into Sea.
This realm relies heavily on resources and patience. One must continuously refine Heaven and Earth Primordial Qi or other precious materials to expand the scale of their Primordial Sea until it is complete.
The Thunder Sound Realm, on the other hand, is about tempering the physical body and opening its apertures.
One must cultivate their physical body to the state of "Jade Bone Thunder Sound, and Blood Qi like wolf smoke"—where their muscles and bones resonate like thunder and their Blood Qi surges like a signal flare. Only then is the realm considered complete.
As for the Divine Communication Realm, its cultivation is even more arduous.
One must open their Spirit Platform, ignite a Divine Fire, and give birth to their Divine Sense.
