Short answers, pulled from the story.
A mandarin orange, Citrus reticulata, is a small, oblate citrus fruit treated as a distinct species of orange. It is sweeter and stronger in taste than the common orange and has a thin, loose peel with little white mesocarp, making it easy to peel and split. The roughly spherical sweet orange is itself a mandarin-pomelo hybrid.
The wild mandarin is one of the pure ancestral citrus species and evolved in a restricted region of South China and Vietnam. It was domesticated at least twice, in the north and south Nanling Mountains, from separate wild subspecies.
The ancestral mandarin orange was bitter, but most commercial mandarin strains derive from hybridization with the pomelo, which gives them sweet fruit. Domestic mandarins also have far lower citric acid than their wild relatives, in some cases almost 90-fold lower.
The custom goes back to the 1880s, when Japanese immigrants in Canada and the United States began receiving Japanese mandarin oranges from family back home as New Year gifts. The tradition spread to the non-Japanese population, and the fruit merged with the European Christmas-stocking story of Saint Nicholas, with oranges standing in for gold balls.
U.S. imports of Japanese oranges were suspended during World War II due to hostilities with Japan. After the war, they were among the first Japanese goods allowed for export, but residual hostility led to rebranding them as Mandarin oranges instead of Japanese oranges.
In 2024, world production of mandarin oranges, reported together with tangerines and clementines, was 52 million tonnes, led by China with 51% of the total. Turkey produced two million tonnes, with Spain and Egypt as other significant producers.