The figure of 6 million dead European Jews was first mooted in the late 1890’s by the Zionists. There were many advertisements for Jewish colonists for Palestine using this figure. Originally it supposedly referred to Russian Jews at risk but over time 6 million European Jews killed was the mantra by the Zionists to gather support for their colonisation of Palestine.
Tens of millions died at the hands of the Germans and a lot were Jews but more were Muslims and Christians and no doubt even some atheists.