There were two types of camps - Concentration Camps and Extermination Camps.

Concentration camps were a widely known thing to exist in Germany even before the onset of war in 1933 and were even documented in German newspapers. And German citizens knew very well that they existed and that Jewish people were being relocated outside the country or the camps.

The SS also started working towards taking control of the concentration camps and begun turning them into extermination camps before the war. Though they were relatively smaller in comparisons (2100 prisoners vs 100,000), but those numbers increased about tenfold before the war broke out.

There were also journals and writings from people in the late 30s where they wrote how they knew that people were being sent to the extermination camps to be killed but they didn't know the method of killing (gas) yet. Not to mention news clippings of the Hamburg bombing in 1943 which was viewed as retribution against Germans for crime against Jews (so they knew crimes were happening).

And that the extermination camps and the "legal" system that supported it had enveloped not just the federal government but municipal and local governments as part of it's legalizing system (similar to how colonial outposts and their judiciary would legalize colonization as just business).

Most historians agree that people knew and if they didn't, then it was willful ignorance to try and ignore what the government was doing.