Short answers, pulled from the story.
Northern Transylvania became part of Hungary on the 30th of August 1940 following the signing of the Second Vienna Award. This agreement transferred the territory from Romania to Hungary under pressure from Germany and Italy.
The 1930 Romanian census registered a population of 2,393,300 for the region while Hungarian authorities recorded 2,578,100 people in their 1941 census. These figures show stark differences depending on whether nationality or language was used as the primary metric.
Excluding Szekely areas, 127,377 Jews were deported to Auschwitz from Northern Transylvania while only 19,764 returned home. Most deportees were exterminated in the Auschwitz and Birkenau camps with just over 800 survivors.
Soviet forces occupied the territory by late October 1944 after King Michael's Coup caused Romania to leave the Axis powers. The Romanian Army then fought Nazi Germany and its allies while regaining Northern Transylvania before Soviet units expelled the Romanian administration in November 1944.