Not all Jewish victims of the Holocaust were killed by gas or executed. It is estimated that at least 1,000,000 died through privation: starvation and disease caused by imprisonment in appallingly overcrowded Eastern European ghettos.

The systematic killing of Jews began with the invasion of the Soviet Union in June 1941. Special SS and police killing units called the Einsatzgruppen accompanied the invading German army to shoot Jews (men, women and children) and suspected communist officials (adults only). The use of gas (first gas vans, later gas chambers) started near the end of 1941. The Einsatzgruppen started using gas vans to kill Jews because the shootings became too psychologically taxing for the perpetrators. Around 1,500,000 Jews were killed by the Einsatzgruppen in the occupied areas of the Soviet Union.

Between December 1941 and January 1945 an estimated 2.8 to 3 million Jews were gassed upon arrival at one of the six extermination camps that were specifically built for the destruction of the Jews in German-controlled territories. All six of these camps were located in Poland. Gas vans were used in the extermination camp at Chelmno. The five other camps had gas chambers and they were (in chronological order of start of operation): Auschwitz, Belzec, Sobibor, Treblinka and Majdanek.

An additional 200,000 Jews died in Auschwitz through disease, starvation and gassing after having been initially spared to perform forced labour.

At least 300,000 Jews died in non-extermination camps through disease, starvation and executions.

The killing in gas chambers in the extermination camps was a streamlined process that could swallow up tens of thousands of victims a day when all six camps were operating simultaneously. The large gas chambers at Auschwitz-Birkenau alone could each hold 2,000 people and kill them in twenty minutes and its crematoria could burn between 4,000 to 8,000 corpses a day (estimates by eyewitnesses vary). The way a death camp worked was: the train arrives, the people are unloaded, they are told they are going to take a shower and to undress, they are herded into the gas chambers, they are all dead and buried/burned before the sun goes down. Jewish prisoner details called Sonderkommandos were forced to empty the gas chambers and dispose of the bodies, initially by burial in large pits, later by burning.

As to who thought up this whole thing: the gas chambers and the concept of telling the victims they were going to the showers in order to get them to enter the gas chambers voluntarily and without resistance, a technique that significantly contributed to the speed at which the killing operation could do its deadly work, were actually pioneered between 1939 and 1941 in the "euthanasia hospitals" in Germany and annexed Austria, where hundreds of thousands of disabled German and Austrian citizens were killed in this way. The gassing of disabled people was (officially) halted in 1941 after church protests and many of the personnel of the "euthanasia" centres were recruited to go work in the death camps at Belzec, Sobibor and Treblinka.

Your last question wasn't addressed, and as I know a reasonable amount on Unit 731 and quite a bit on Nazi experimentation I shall attempt to do so.

Unit 731

Unit 731/The Epidemic Prevention Research Laboratory/the Ishii Network was officially established in 1936. Lt Gen Shiro Ishii was given clearance by the high-level Japanese authorities to possibly invent, develop, and ultimately deploy chemical and biological weapons. This was largely done out of a fear that the Soviets too possesed chemical and biological weapons. Japan's biological and chemical weapon program with regard to Unit 731's position was like one big family tree. Unit 731 was at the top, and many other units which propped up across Asia as the war continued branched off. Additionally each Unit was also seperated into different sections I.E one section researched biological warfare, another section trained medical personnel. I think a dozen different Units have been confirmed to have existed, and they were all under the command of Lt Gen Shiro Ishii. This equates roughly to about 10,000 personnel. The number of people who fell victim to Unit 731's experiments is difficult to estimate. Maj. Gen. Kawashima Kiyoshi, former head of Unit 731's first, third and fourth sections, during his trial at Khabarovsk, USSR admitted

I can say that the number of prisoners of Detachment 731 who died from the effects of experiments in infecting them with severe infectious diseases was no less than about 600 per annum

We have a figure of about 3000 of "indoor" experiments. The number of field experiments is even more difficult to estimate. It is unknown the ratio between Chinese who died in "natural" epidemics to the epidemics Unit 731 caused. What is considered to be the largest recorded biological attack on a Chinese city carried out by any Unit was that on Ningbo in 1940. Two subsequent investigations concluded 106 people died. Though in a rather ironic case, a biological attack misfired and over 10,000 Japanese troops fell ill with the plague.

Nazi Experiments

Nazi medical killings arose from Action T4 (1939-41), the euthanasia programme in which tens of thousands of mentally ill Germans were killed. The role of doctors in Action T4, and the way doctors learned to dehumanize the mentally ill as well as how they learned to rationalize killing, heavily influenced the role of doctors in the Nazi concentration camps. It was doctors who selected those fit and unfit for work on arrival to the camps. It was doctors who selected those labourers who were fit or unfit to continue working in the camps. In effect, Nazi doctors were central to the extermination of undesirables in the sense they chose who lived and who died. From 1941 onwards, whenever a medical block at Auschwitz was overcrowded SS doctors "euthanized" prisoners with phenol injections. Like with Action T4, the doctors rationalized this was merely mercy killings coupled with toxic antisemitism. Similarly, Mengele's experiments are linked to his involvement in selecting the fit and unfit arrivals to Auschwitz and his role in "euthanizing" prisoners with phenol injections.

Along with Unit 731, it's difficult to find accurate figures for the ratio for people who died in gas chambers, from disease and from experiments. It is likely in the low thousands.

The Difference

The experiments at Unit 731 were conducted on a systematic and consistent level, and they were known by members of the Imperial family (Prince Tsuneyoshi Takeda personally visited Unit 731 and Emperor Hirohito approved of the establishment of Unit 731), the Japanese army leadership, and possibly several civilian members of the Diet. Unit 731 was also funded directly by the Japanese state, with the funds allegedly making it the most technologically advanced bio-chemical research facility in the world. These experiments were not done as part of some plan to exteriminate the Chinese, they were viewed purely as a means of military necessity.

In contrast, Nazi experiments were often carried out by individual doctors without directive from the Nazi state (aside from the Sterilization experiments which were given the green light by Himmler). Few knew about them, even SS guards in the same camp as Mengele had little idea who he was. Were Nazi experiments comparable to Unit 731? The psychology of the doctors involved were similar in the sense they dehumanized their victims (Maruta and Seuchenverdächtige Juden), however the scale and reasoning behind the killings are vastly different. As I mentioned previously, Unit 731 experiments were conducted in an attempt to perfect bio-chemical weapons. Japanese experiments occurred both within and outside of the different Units. Nazi experiments were confined to the concentration camps. Nazi experiments were also interlinked with the administration of the camps as well as motivation brought about by the toxic racial elements of Nazi ideology. Nazi experiments, specifically Mengele's, were psuedoscience at best whereas with Japanese experiments there was a specific goal. Yet the doctors managed to persuade themselves that this was legitimate research.

Sheldon H. Harris, Factories of Death: Japanese Biological Warfare, 1932-1945, and the American Cover-up, 2002

Yuki Tanaka, Hidden Horrors: Japanese War Crimes in World War II, 1996

Tsuneishi Keiichi, Unit 731 and the Japanese Imperial Army’s Biological Warfare Program, 2005

Robert Jay Lifton, The Nazi Doctors: Medical Killing and the Psychology of Genocide, 1988