Unfortunately you asked several questions and are only allowed one per post, so I had to edit.
Night by Elie Wiesel is the story of Elie's time in a concentration camp during World War II. In chapter six of the story, Elie and the others are moving from one camp to another. A witness in this story is one who can testify to what is happening, and there are many in this chapter. The events of this chapter demonstrate the dehumanization which is happening to the Jews, and the witnesses in this chapter testify to that.
First and most obvious are the SS men who are guarding, moving, and abusing their prisoners. Though they would testify differently about the events, nevertheless they are witnesses. As always, they treat the Jews like animals.
Elia and his father are also obvious witnesses to dehumanization, as Elie wrestles with his desire to leave his father behind as a hindrance and a nuisance, a weight he would be better off without.
Another witness is a young man names Zalman.
He had worked in the electrical material depot in Buna. People mocked him because he was forever praying or meditating on some Talmudic question.
He marches with the others until he is forced to stop because of his explosive stomach and, we assume, is trampled by his fellow marchers. He, too, is a testimony to the devaluation of life.
Rabbi Eliahu and his "beloved son" are also witnesses to the increasing sense of dehumanization. The son does what Elie has considered, and the Rabbi wants to believe (perhaps really does believe) that his son accidentally got separated from him. This son turning against his father is clear testimony of the devaluing of life and relationship that is happening in this chapter.
Juliek is a violinist, and he is the most poignant witness in this chapter. In a setting and environment of animals being herded and trampled, Juliek testifies that the Jews are still men, not animals. He manages to find his violin, and he plays a bit of a Beethoven concerto, something forbidden to the Jews.
All I could hear was the violin, and it was as if Juliek's soul had become his bow. He was playing his life. His whole being was gliding over the strings. His unfulfilled hopes. His charred past, his extinguished future. He played that which he would never play again. I shall never forget Juliek. How could I forget this concert given before an audience of the dead and dying? Even today, when I hear that particular piece by Beethoven, my eyes close and out of the darkness emerges the pale and melancholy face of my Polish comrade bidding farewell to an audience of dying men.
Juliek is a witness to something positive, to the fact that, despite the degeneration of the prisoners from men into animals, the human soul can still express itself through music. Elie is a witness to the fact that, despite the brutal dehumanization by the Germans, the prisoners are still capable of being moved by beauty.
All of these witnesses testify, in words, actions, and music, to the events happening around them.