The lithograph was a genuine Chagall that was stolen. "That was stolen" is a
modifier in this sentence.

well the central charater is kaneki so kaneki but in tokyo ghoul re kaneki has a new persona so i guess his new persona trying to be stronger

it is the background information on the characters and setting explained at the beginning of the story. the exposition will often have information about events that happened before the story began. the exposition is often the very first part of the plot.