They were built by the SS, using slave labour.
The Nazi extermination camps (that is, camps built solely for the purpose of killing) were all built in 1941-42. There were 6-8 such camps. Please see the related questions.
Concentration camps were used for forced prison labor, while extermination camps were built to kill all prisoners.
Nazi leaders wanted to keep the existence of extermination camps secret.
Ghettos, concentration camps and extermination camps.
No, but it had the Mauthausen group of camps.
Nazi leaders wanted to keep the existence of extermination camps secret.
The extermination camps were top secret.
All the main extermination camps were in Nazi-occupied Poland. There were also two in Belarus. Please see the related question.
All the main extermination camps were in Nazi-occupied Poland. There were also two in Belarus. Please see the related question.
Concentration camps, Extermination camps and Death camps
The key distinction was between extermination camps and labour camps ("ordinary" concentration camps).
it started at the end of 1941 and the last was in 1944.