Their progenitor left a seed in Famed Bamboo that eventually grew into a tree, called Adamantine.
It was both strong and magical. For example, if one were to break a branch and leave it somewhere else. This branch would grow in an insane rate and absorb the energy and water nearby, leaving nothing left for the land.
After it became big enough, it could stay or fly back to Famed Bamboo. It would wither then and all of the absorbed energy would be taken by the mother tree.
Because of this, the original one continued to become stronger with many heaven-defying techniques. Numerous Eternals and previous emperors in this system viewed it as a guardian senior.
This was indeed the case for millions of years. The system grew under its protection. Many powerful intruders have come only to leave empty-handed before the might of the tree.