According to Wikipedia, they used a lot of different approaches. Zyklon-B was abundant, as it was used as an insecticide, but was dramatically more poisonous to humans. 4kg of the poison can kill 1000 people. As far as I understand, it proved itself to be the most efficient method for killing a lot of people reliably

This is meta but…

This post demonstrates the utility of having an r/askhistorians equivalent on lemmy. I seem to remember them being quite outspoken against Reddit’s bullshit, but I’m not sure if they went anywhere.

First: Zyklon B was mainly used in Auschwitz, many other camps did use CO or engine exhaust gases.

Nazi death camps used shower heads to introduce a gas into the gas chambers

No, the showers were simply fake. Zyklon B was inserted from above into metal pipes so it would fall down into the gas chamber. You can see an animation here: https://www.zdf.de/dokumentation/terra-x/die-gaskammern-in-auschwitz-birkenau-creative-commons-100.html

and likely relatively expensive

It was widely used for killing insects for example on ships, cooling houses, mills or other in other storage facilities. In fact the overwhelming part of Zyklon B going to concentration camps was indeed used to kill fleas or lice and not for killing humans. The price was 4.55 RM in 1943 per kg which is roughly the same in Euros today.

It also meant that the gas chambers had to be aerated for a number of minutes

30 to 40 minutes

Why didn’t they simply use CO2

Zyklon B was again a mass product with the corresponding production capacities already there and not expensive. Also it is estimated that around 4kg of Zyklon B are able to kill 1000 people. It is also fast acting and thereby simply very effective in that sense. Plus it was also simple to transport and store as it came on a carrier material, so pressure gas bottles were not necessary but simple metal containers were used.

They did sort of use CO2/CO in the earlier days, in the form of poorly maintained van trucks that would drive around the camps with their exhaust piped directly into the back of the van. That was slow and inefficient though, as the truck had to run long enough to actively displace all the oxygen within the volume of the van.

Zyklon B from its Wikipedia page was intended as a pesticide so there was already a large industrial supply lying around. It works on a cellular level and will cause widespread cell death within 2 minutes of inhalation at extremely low concentrations, making it ultimately much faster with less maintenance.

I'm not a historical expert, but the deaths caused by Zyklon B are also described to be much, much more violent than simply passing out from oxygen deprivation- and I could posit that the Nazis, viewing jews et al. as not human, would have preferred such a death for them.

They did not only use Zyklon B. They also used carbon monoxide from bottles and engine exhaust. But they considered Zyklon B more effective, especially the commandant of Auschwitz. They could just drop the pellets into the chamber, no need to handle large bottles, keep a hose connected, run an engine etc.

This is pure speculation, but I can also imagine that using a pesticide to exterminate Jews and their various other enemies/victims would appeal to Nazis.

During Operation Reinhard they mostly used carbon monoxide.

HCN was, and continues to be an industrial product and is pretty cheap. it's used on industrial scale in some plastics precursors manufacture, like PMMA (acrylic glass) or cyanoacrylate glue among others. currently flow processes allow it to be generated and consumed in adjacent reactors, minimizing amount actually generated at any time, but it was not always the case

it's low-boiling liquid (bp +25C) meaning simple transport compared to gases like CO or CO2. in the era some parasites, like louse were common and HCN was used for de-lousing clothing, and some derivatives were used as a general insecticide for fumigation. that's partly because there were no better agrochemicals developed yet. it's dangerous for all life and by its mechanism works rather quickly (minutes) and is rather potent. CO2 is neither

Possibly because the Zyklon B was faster acting?

I cannot remember when or where I heard or read about this, so take it with a grain of salt. But I recall hearing something about the people who had to remove the bodies from trucks where people had been gassed with CO2 finding the corpses so gruesome and agonising, as opposed to more peaceful looking bodies of people killed with other gases, that they stopped using it. Presumably the difference was because of how slow or quick the death was.

I doubt they would have cared the tiniest bit later on when it was other concentration camp victims who had to clear the bodies.

I have to admit that I'm curious about something similar; the death camps and ots mechanism seemed highly inefficient, but the fact that I have to preface any questions on the topic with how much i detest the actual concept made me conclude that it's probably best not to ask. That's the annoying fact about being interested in finding efficient technical solutions while also hating nazis and everything they stood for.