Short answers, pulled from the story.
Estimates range from approximately 2.8 million to 3.9 million deaths out of nearly six million Soviet soldiers captured. Historian Viktor Zemskov estimated around 3.9 million dead from 6.2 million captured, while Christian Streit estimated 3.3 million and Dieter Pohl estimated 2.8 to 3 million, each working from slightly different capture figures.
Germany signed the 1929 Geneva Convention and generally observed it with prisoners from other countries, but the OKW declared it did not apply to Soviet prisoners. The Soviet Union had offered to abide by the Hague Convention if Germany did likewise, but Adolf Hitler rejected that offer several weeks after the invasion began.
The Commissar Order, issued before the June 1941 invasion, directed German forces to execute captured Soviet political commissars and civilian political functionaries. More than 80 percent of front-line German divisions on the Eastern Front carried it out, with an estimated 4,000 to 10,000 commissars killed. The order was rescinded in May 1942 after it was judged to be stiffening Soviet resistance.
By the end of the war, 1.4 million Soviet prisoners of war out of a total of 2.4 million were serving in some kind of German auxiliary military unit. These included ethnic-minority legions, anti-partisan battalions, and the Trawniki men, who helped suppress the 1943 Warsaw Ghetto Uprising and staffed extermination camps in German-occupied Poland.
Soviet prisoners of war did not receive any reparations until 2015, when the German government paid a symbolic amount of 2,500 euros to the few thousand still alive. They had previously been excluded from the Foundation Remembrance, Responsibility and Future, which administered earlier German reparation payments.
Freed prisoners were processed through filtration camps; according to official statistics, 57.8 percent returned home, 19.1 percent were remobilized, 14.5 percent were drafted into labor battalions, and 6.5 percent were transferred to the NKVD. Former prisoners were not recognized as veterans and faced discrimination until Russia equalized their status with other veterans in 1995.