This page of the NieR Automata game guide contains the walkthrough of Chapter 8 during Route B. This guide describes what to do in 9S' memory bank to complete Chapter 8.
Chapter 8 in Route B is entirely new as a result of the events that occur at the end of Chapter 7. While 2B is busy trying to reach the Copied City, you have to explore 9S' memory with the intention of returning to the real world.
At first glance, 9S' memory bank looks like a maze, but you don't have to worry about getting lost in it, because there will always be a single path leading to the next destination. When exploring the virtual reality, stop at the white squares and hold down the hack button.
This will trigger text scenes. There's no special way to interact with them. You just have to scroll through the text.
Continue to reach consequent white squares and read the walls of text until you begin to experience image distortion.
The next cut-scenes will play, from which you will learn that you arrived at the point during which, in Route A, 2B took part in the Adam boss fight.