There are reports of the Allied generals being horrified (obviously rightly so) when they arrived at the Nazi concentration camps, and not expecting to see the level of depravity they witnessed. With that being said, what did they expect to see when they arrived. In other words, how much of what the Nazis did there were they able to keep secret from the Allies? Were the Allies expecting to find a bunch of surviving Jewish prisoners? Or did they know most of them would be dead, but maybe not the level of torture the prisoners had to endure as well? Also, how much of what went on in them were they able to keep secret from the German population?
/u/commiespaceinvader has previously answered:
/u/kieslowskifan has previously answered Did the Germans who lived near concentration camps really not know that they were there?
/u/estherke has previously answered:
I have previously answered the question about the German people's knowlege of the Holocaust in general https://www.reddit.com/r/AskHistorians/comments/cs9aor/was_the_holocaust_know_by_the_majority_of_german/execicl/?context=3
Of course, the knowledge about specifically the camps would be even less well-spread.
Specifically the extermination camps (as opposed to "mere" concentration camps) had the highest degree of camouflage: http://holocaustcontroversies.blogspot.com/2018/12/whats-there-to-hide-camouflage-and.html
Thus, the Red Cross was strictly forbidden to inspect Auschwitz and Auschwitz-Birkenau, this is documented by the Nazis themselves:
Certainly, the information about some of what was going on in various camps was appearing in the Western and Soviet press for years, at least since 1942. Take the Polish Govt-in-Exile's 1942 report "The Mass Extermination of Jews in German Occupied Poland" https://www.msz.gov.pl/resource/e7497fea-f446-4f82-80b1-169d609d697a:JCR or the 1944 report of the escaped Auschwitz inmates Wetzler and Vrba. There were thousands of various such articles, naming Treblinka and other camps.
But with all such reports, and the war going on, I speculate (and I say so explicitly) that the public may have become numb to the facts of the reports themselves. So when the reality hit after the liberation of the camps, that was still a shock - not mere theoretical words on paper but the real horrors.
Just as an example, as the Soviets were moving towards Auschwitz, they were liberating its smaller subcamps one by one and we have the liberation reports. You can read some of them here and see the progression and, sometimes, the emotions: