Short answers, pulled from the story.
Lothrop Stoddard published The Revolt Against Civilization: The Menace of the Under-man in 1922. He used this term to describe people he believed could not flourish due to inferior heredity.
Heinrich Himmler directed the Reich Security Main Office to produce the brochure Der Untermensch in 1942. Nearly four million copies were printed and distributed across German-occupied territories.
Nazi ideology categorized Jews, Roma, Slavs, and people with disabilities as Untermenschen who were to be exterminated. Poles occupied the bottom of the Slavic racial hierarchy established by the Nazis.
Early Nazi plans summarized as Generalplan Ost envisioned ethnic cleansing and extermination of no fewer than fifty million people from conquered European territories. Planners considered Ukraine's chernozem soil a particularly desirable zone for colonization.
Hitler signed an edict officially starting Aktion T4 euthanasia programs on the 1st of September 1939. Carbon monoxide was first used to murder disabled patients through this initiative.