Good question! So the extent of awareness of camps will always remain blurry, and here's why. Places like ghettos were common knowledge, many people witnessed Jewish families being sent to ghettos- before the camps. Because of this there was an understanding of "they're going somewhere".

When the camps were in use the Nazis made sure to cover up as much as they could, keeping citizens blind to the true horrors. If you can recall the American internment camps used to gather the Japanese- Americans, that is likely what the avg. German thought was happening. There was of course propaganda that claimed the Nazis were doing far from the truth.

Now, for places like France and Belgium. As I mentioned before, ghettos weren't very secretive. There were most obviously rumors about the camps, though. These grew from those who escaped the camps, or saw something they shouldn't have. People outside of Germany and outside the Nazi perspective could have easily believed these rumors, as people often side with rumors against the enemy. Another point is that when the camps were discovered/liberated many people couldn't wrap their minds around the magnitude. So it could also be argued that others didn't believe the camps were real until the truth and horrors were staring them in the face.

All in all, people had figured out that wherever the Jews were being sent to they were not coming back from it.