Short answers, pulled from the story.
Sweden was the primary source, accounting for 43 percent of all iron ore Germany imported between 1933 and 1943. In the final peacetime year, Sweden shipped 9 million of the 22 million tons Germany received in total.
Grand Admiral Raeder, head of the German navy, declared it would be utterly impossible to make war without access to Swedish ore. Germany's own mines produced low-grade ore that had to be blended with high-quality Swedish material to manufacture workable steel.
The Norwegian Corridor was Churchill's name for a 1,600-kilometer sheltered sea lane along Norway's western coast, protected by the Skjærgård, a chain of roughly 50,000 glacially formed rocks and skerries. German ore ships loaded at Narvik used this passage to travel inside Norwegian territorial waters and avoid Royal Navy interception, especially during the winter months when the Baltic was frozen.
Churchill, as First Lord of the Admiralty, first developed Project Catherine, a scheme drawn up by Admiral of the Fleet William Boyle to send a British fleet into the Baltic. He later pushed for mining the Norwegian coast, leading to Operation Wilfred on the 8th of April 1940, which placed one minefield in the mouth of Vestfjord leading directly to Narvik.
Sweden ended the iron ore trade with Germany by November 1944. Transport through the port of Luleå had ended the same year, following sustained Allied pressure that included a September 1943 agreement on ball bearing exports and diplomatic discussions at the Moscow Conference of Foreign Ministers in October 1943.
Iron mining in northern Sweden began to recover in the autumn of 1946 with shipments to England. The mining company LKAB signed a contract with Bethlehem Steel that same year. By the early 1950s, annual production had returned to the levels of the best years of the 1930s.