Gather your armies
Whether they be called minor realms, hidden realms, secret realms, pocket dimensions or anything else, the concept was the same. It was basically a separate pocket of space, disconnected from the main universe. Sometimes they formed naturally, and in some cases, a very high leveled cultivator could open them up on their own.
These separate dimensions had their own ecosystem and existed in their own sustained balance. Sometimes, this ecosystem was extremely hazardous and could not support life. But in cases like this, such realms often give birth to extremely precious and rare treasures, whether they be in the form of precious ores, liquids, gasses or anything else. Most of the time, however, these realms had a very stable environment, rich in spiritual energy.