Short answers, pulled from the story.
Zealand had a population of 2,319,705 on the 1st of January 2020, making up approximately 40% of Denmark's total population. Copenhagen, located mostly on the island's eastern shore, held between 1.3 and 1.4 million of those residents in 2020.
Zealand covers 7,031 square kilometres, making it the largest island in Denmark proper and the 13th-largest island in Europe by area. It is the 4th most populous island in Europe.
The exact origin of Sjælland is uncertain. The leading hypothesis links it to a reconstructed Old Norse word, *selha-, meaning either "seal" or "deep bay, fjord", combined with an ending meaning "resembles". Some scholars believe early sailors named the island after the Roskilde Fjord, a narrow inlet leading to the ancient settlement at Roskilde.
Zealand is linked to Funen and Jutland by the Great Belt Fixed Link, and to Sweden's Scania region via the island of Amager and the Øresund Bridge. In the south, the Storstrøm Bridge and Farø Bridges connect it to Falster and onward to Lolland. The Fehmarnbelt Tunnel, under construction at the time of writing, would extend this corridor to the German island of Fehmarn and mainland Europe.
According to the Gylfaginning, the goddess Gefjun tricked Gylfi, the king of Sweden, into letting her take a piece of his land. She used her sons, transformed into oxen, to haul the territory to Denmark, where it became Zealand. The void left behind reportedly filled with water and became the Swedish lake Mälaren, though some identify lake Vänern as the better geographical match.
The first section of railway on Zealand opened in 1847 between Copenhagen and Roskilde. It was constructed by Det Sjællandske Jernbaneselskab, a company that operated from 1847 to 1888.