What are Tomura's extra hands exactly? What are they for and what are they made for? are they artificial or made of organic material, is it known?
It is revealed in Chapter 222 that the hands are from the remains of his family.
It is not known what his extra hands do or are made of. This is neither addressed in manga or anime.
However, the hand on his face has been addressed. He calls it "Father" and is visibly distressed if it is taken off him.
But there is no mention about the material or the backstory of the existence of all those hands in the first place.
I believe that there is a scene where Tomura watches a hand in a pool of blood and calls his parents, so I would presume his parents were killed and the hand on his face is his father’s. I know it sounds creepy, but it would make sense.