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Hi there -- here are a couple of older posts from u/commiespaceinvader that may help to answer the question of how historians arrive at the number of Jewish deaths in the Holocaust (between 5.4 million and 6.2 million, depending somewhat on how you count):
and
The sources Commie cites are:
On the Höfle Telegram: Peter Witte, Stephen Tyas: A New Document on the Deportation and Murder of Jews during „Einsatz Reinhard“ 1942. In: Holocaust and Genocide Studies 15, 2001, pp. 468–486.
On the Korherr Report: Gerald Fleming: Hitler und die Endlösung. Wiesbaden/München 1982.
On Auschwitz: Franciszek Piper: Die Zahl der Opfer von Auschwitz. Staatliches Museum Auschwitz, 1993.
Wolfgang Benz (Hrsg.): Dimensionen des Völkermords – Die Zahl der jüdischen Opfer des Nationalsozialismus., München 1996.
Raul Hilberg: The Destruction of the European Jews, Yale 2002.
Saul Friedlände: Nazi Germany and the Jews, Vol. 1 and 2.
Doris L. Bergen: War & Genocide: A Concise History of the Holocaust.
Christian Gerlach: The Extermination of the European Jews, 2016.
Nicholaus Wachsmann: KL. A history of the Nazi Concentration Camps, 2015.
No. /u/commiespaceinvader has a great answer here addressing the way that deaths in the Holocaust have been calculated, and adjusted, but certainly awareness that many deaths were not in the camps, but through methods such as the Einsatzgruppen was there from the start.