Actually, concentration camps came very early. And the Nazis were very open about these concentration camps, because they figured they would serve as a deterrent. However, they weren’t initially used to house Jewish people, but mostly for political opponents, like socialists. Later came the intellectuals, journalists, teachers, scientists, etc. And of course other undesirables like gay people.

Extermination camps, however, were a relatively late addition. And they were kept secret. The systemic extermination of Jewish people began after the Wannsee Conference in 1942, where Nazi leadership discussed the Final Solution to the Jewish Problem. Under Operation Reinhard they started building the six extermination camps:

• Chełmno
• Bełżec
• Sobibór
• Treblinka
• Auschwitz II-Birkenau (Auschwitz I was an existing concentration camp, Auschwitz II was built a few kilometers away from the main camp)
• Majdanek (Majdanek was converted into an extermination camp. It started as a concentration camp mostly for POWs).

The first 4 camps were built specifically and solely to kill people.