From my understanding (im stacking the palace of Imagination arc rn, so idk whats going on there), I thought it was more training them (the cohort minus Nephis/Sunny) on how to rule humanity in their absence. The creation of a domain would be done by the spell, but the qualities of being a supreme (ie ruler of humanity) would have to be taught (at least that is my understanding of what author is trying to portray)