I remember years ago I watched "The Boy In The Striped Pyjamas" and one of the things that struck me the most about the movie was how no one in the movie except for the military seemed to actually know that the Jews were killed in the Concentration Camps, and instead thought it was kind of like a pleasant temporary home until they were expelled from Germany.
Is that part of the movie historically accurate? If not, how much did the citizens actually know?
I apologize if my English is flawed, it is not my primary language.
The user and expert on this topic u/commiespaceinvader wrote an answer to that question earlier. You can read it here if you like.
This has been a somewhat contentious issue in the history of the Holocaust and WWII. Sometimes it comes down to the messiness of language. When we say 'concentration camp' what do we mean? The Ghettos? The literal concentration camps where Jewish populations were detained? The labor camps? The death camps?
Generally I assume most people mean 'all of the above' cause we've kind of adopted concentration camp as a catch all term for everything the Holocaust entailed, deportation, incarceration, and extermination. My answer will reflect this. Please specify if you meant concentration camp in a more specific way.
I'm going to just borrow the Amazon blurb for KL: A History of the Nazi Concentration Camps because I think that blurb for Nicholas Wachsmann's book really illustrates the innate absurdity of any claim that includes the words "we didn't know":
In March of 1933, a disused factory surrounded by barbed wire held 223 prisoners in the town of Dachau. By the end of 1945, the SS concentration camp system had become an overwhelming landscape of terror. Twenty-two large camps and over one thousand satellite camps throughout Germany and Europe were at the heart of the Nazi campaign of repression and intimidation. The importance of the camps in terms of Nazi history and our modern world cannot be questioned.
Dr Nikolaus Wachsmann is the first historian to write a complete history of the camps. Combining the political and the personal, Wachsmann examines the organisation of such an immense genocidal machine, whilst drawing a vivid picture of life inside the camps for the individual prisoner. The book gives voice to those typically forgotten in Nazi history: the 'social deviants', criminals and unwanted ethnicities that all faced the terror of the camps. Wachsmann explores the practice of institutionalised murder and inmate collaboration with the SS selectively ignored by many historians. Pulling together a wealth of in-depth research, official documents, contemporary studies and the evidence of survivors themselves, KL is a complete but accessible narrative.
While this issue was once quite contentious, I think we've generally moved the goal post these days. Historians have mostly gravitated toward the same answer after decades of debate after the war: The German people knew quite a lot about the Holocaust, to the point any claim that they didn't know how bad it really was is difficult to believe. They may not have witnesses the full horrors of the gas chambers first hand, but they knew, as much as any public can know what their government is doing, an industrialized extermination was taking place.
They knew Jews were being shipped out to the ghettos. They knew when those ghettos in turn started being emptied. They had sons and even daughters who were fully aware of the death camps. They themselves had pervasive and wide knowledge of the labor camps themselves. You simply cannot move 11,000,000 people and keep it a secret where they're going or that they're not coming back. While I would be wary to point at any specific individual and say "you had to know" I would say we do point at the whole of German society at the time and say "there's no way people didn't know."
How willing and supportive they were of the Holocaust is a whole other kettle of fish and is where the debate today is more focused.
There's a lot of books on this that I think are worth reading, though all the ones I know of are in English. I would highly recommend Wachsmann's book, which I mention above. Additionally:
Tony Judt, Postwar: A History of Europe after 1945 deals heavily in it's early sections with denazification and the aftermath of the Holocaust. He's got some great analysis on this question.
Robert Gellately, Backing Hitler, which is all about the socio-political dynamics of the Nazification of Germany and considers questions of coercion, consent, and cooperation with the persecution of undesirables and the Holocaust.
I also rather like What We Knew: Terror, Mass Murder, and Everyday Life in Nazi Germany. It's a nice little oral history by historian Eric Johnson and sociologist Karl-Heinz Reuband. It's an oral history and I'm a sucker for them but unlike a lot of oral histories that just slap their testimonials in front of you, Johnson and Reuband include a fair bit of analysis to help frame the stories people tell in the book.