Strictly speaking, Foundation Establishment is the first realm of the Immortal Path, which lays the foundation and is hence called Foundation Establishment.
As for Golden Core cultivators, their Spirit Seas have condensed within, and when Golden Core is formed, they become a Gold Core Spirit Sea life. They are no longer held back by fate, and have taken the key step towards transcending from 'human' to 'immortal'.
Thus, Foundation Establishment cultivators can only utilize flying tools, while Golden Core cultivators are able to cast off Earth's gravity, flying through the sky and earth, relying on their Spirit Sea's Golden Core.
Advancing from the Foundation Establishment stage to the Golden Core stage is much more difficult than advancing from the Qi Cultivation stage to the Golden Core stage.
