Since 6M people got killed, how come there weren’t any fights between those who were picked up to be sent to the camps and the germans? Or were there but have they been forgotten by history?
Not to preclude further answers, but many did.. As /u/Commiespaceinvader explains in the linked thread, examples of Jewish resistance to the Third Reich were varied and widespread, and the question of resistance in the face of certain death can be far more complicated than we might assume.
/u/commiespaceinvader discusses the methods of the Final Solution, the nature of resistance, and other matters concerning the methods of the Holocaust, in this Monday Methods thread as well.
To add to the above comment, they did when they could.
To start with, no one knew what was happening to the Jews when they were being "relocated". So a lot went believing they would be safe. By the point people knew what was happening, most Jews were in ghettos with very little access to money or weapons. There was little they could do. Any who ran or hid were killed.
Despite this, there were several attempts to escape ghettos. The most famous is the Warsaw Ghetto Uprising in 1943. It led to around 12,000 Jews being killed. Most were burned alive as the whole ghetto was destroyed.
Here's a good book on the subject. Edelman, Marek (1990). The Ghetto Fights: Warsaw, 1941–43. London: Bookmarks Publications
There are also the memoirs of Władysław Szpilman, who survived the ghetto. His memoirs were made into the film The Pianist that is actually quite an accurate representation of what happened.
The other poster has great info on insurgency in the camps.
What I also found interesting is the way the camps were organised. The Germans pitted Jews against Jews. Some were leaders and given more food and privileges in return for helping to control the other Jews. They accepted as a means of survival. It meant most Jews didn't trust the other Jews in the camps.
A great book to read that explains the phychology of the camps is Primo Levi's If This is a Man. I never really understood the camp experience until I read the book. He was an Italian Jew that ended up in Auschwitz.
(Edited to correct spelling)
Also, two of the six death camps in Poland were essentially destroyed after mass prisoner revolts. There were revolts in Treblinka and Sobibor and between both approximately 120 people survived the escapes. In Birkenau too the sonderkommando revolted and destroyed one of the gas chambers. This did not stop the killing by any means but the charge of “going like sheep to slaughter” that is often presented is inaccurate. There were large uprisings in multiple ghettos and a number of Jewish partisan units. Even those who did not practice violent resistance enacted spiritual resistance each day. Even those who “went quietly” almost always did so while enacting inner resistance, by holding their children, by praying, by singing, by just continuing to be human until life was ripped from them