I answered most of your question in the process of responding to a related one:

https://www.reddit.com/r/AskHistorians/comments/xyqg58/comment/irj4zya/

There were multiple decision points. In preparation for the invasion of the USSR, the Einsatzgruppen were instructed to kill a Jewish men 15-50 years old. This process escalated so that, by August 1941, whole Jewish communities were being eradicated. At the same time, the toll that these mass shootings were taking on the men tasked to them was becoming clear. When Himmler attended such a shooting in Minsk in August 1941, he was warned by General Bach-Zelewski of this matter.

Second, mass overcrowding of Polish Jews and Jews from the Reich within the Generalgouvnement raised alarms to public health officials that epidemics could spread beyond ghetto walls into the general community. When prospects of a longer war became clear and plans of mass expulsion to other places, such as Madagascar, proved impossible, the decision was made, likely in the fall of 1941, to submit Polish Jews to mass extermination as well. Aktion Reinhart was established for this purpose.

There is debate over whether a separate decision was made to condemn to death the remaining Jews of Europe (France, Greece, Hungary, etc.). If it was, it likely came after the declaration of war with the United States, so in the second week of December 1941.

As noted in the above link, Christopher Browning treats this process in exhaustive detail in Origins of the Final Solution.