Short answers, pulled from the story.
The hazelnut is the fruit of the hazel tree and includes any nuts deriving from species of the genus Corylus. Most commercial varieties come from the species Corylus avellana.
Researchers found evidence of large-scale Mesolithic nut processing some 8000 years old in a midden pit on the island of Colonsay in Scotland. The nuts were radiocarbon dated to 7720 plus or minus 110 BP which calibrates to around 5770 BCE.
Turkey was the world's largest producer of hazelnuts in 2023 accounting for 57.7% of total production. World production reached 1.13 million tonnes that year led by Turkey with Italy and the United States as other major producers.
Hazelnuts appear in confections to make pralines chocolate truffles and hazelnut paste products including nougat. Ground hazelnuts combined with chocolate developed around Turin Italy became known as gianduia and subsequently created Nutella and similar spreads.
Raw hazelnuts contain 5% water 61% fat 17% carbohydrates and 15% protein by weight. Hazelnuts account for 93% Daily Value of dietary fat within a 100 gram serving with fat components consisting primarily of oleic acid at 75% total.