During WWII, how much of the German population knew about the Holocaust? I have listened and read a few of Hitlers speeches and don't understand how the people could plea ignorance when Hitler talks of "exterminating Jews from Europe ". Any informed insights would be greatly appreciated.

Estherkes answer is excellent, but as an additional comment I want to add that "the Holocaust" covers several different waves of mass murder (depending on definition). Arguably the euthanasia program was the first stage of the Holocaust. It was a systematic mass killing of disabled and mentally ill patients. The killings was carried out in secret, and relatives to the victims were informed the victims had died by natural causes. However people realized what was going on anyway and this lead to widespread protests. Too many people was involved (doctors, nurses etc) and some of them talked, and relatives to the victims realized that the official causes of death was faked. Even more interesting is that the public protests was strong enough to make the government pause and scale down the program.

Now, the fate of Jews is a different matter. It was totally public how Jews was gradually stripped of their rights until they were removed from their homes and interned in ghettos. What happened in the final stage was not publicized, but given the events of the euthanasia program, people could probably figure it out.