I mean Qyburn isn't actually a maester he got kicked out, he's just Dr. Frankenstein. Yeah the Children of the Forest and Giants are the original inhabitants of Westeros, who worshipped the Old Gods. The First Men came across a land bridge called the Arm of Dorne. It boiled over into a war between the First Men and the Children that lasted millennia. The Children used something called the Hammer of Waters to destroy the Arm of Dorne, and also create the Neck, the swamp that separates the North and South. They eventually reached a peace and had to unite during the Long Night to stop the White Walkers, after which the Wall and Winterfell were built, the Night's Watch started, and the First Men took on the Old Gods. Then when the Andals came over with their religion of the Seven and landed in the Vale, they practically wiped out or interbred with the First Men and whatever remained of the Children were decimated, except for their inability to cross the Neck and integrate with the North, which is why the North keeps the Old Gods and has a higher concentration of First Man DNA than the rest of Westeros. Then much later the Rhoynar land in Dorne but that's not until a few hundred years before the Doom of Valyria.