In Ch.15, the Commandant comes, and the children hide in the refuse and sewage rather than be discovered and killed. Many friends from the village have been "Chosen" for extermination, and the women recite the Kaddish, or prayers, in their memory.
In Ch.16, the inmates settle into a routine where the sole focus is on staying alive. Hannah learns that each time the blokova loses control of her group, she loses a finger. The Commandant returns, and little Reuven is caught, taken away by the Commandant, and not seen again.
Shmuel and Yitzak have a plan to escape with their families in Chapter 17, but it is unsuccessful.
The camp witnesses the execution of six men captured trying to escape in Ch.18. Shmuel is among them; as he is about to be shot, Fayge runs to him and they die together . Yitzchak alone has escaped. Hannah, Rivka, Esther, and Shifre are working in the field when the Commandant comes to "Choose" three for "transport". Hannah is spared, but sacrifices herself so Rivka might live. She walks through the door to her doom.
In Ch.19, Hannah finds herself back with her family in the present. Aunt Eva reveals that her old name is Rivka, and she was saved by her niece Chaya in the camps. By having lived Chaya's life, Hannah better understands her Jewish past, and the importance of memory.
As above in Ch 18 you said that the Commandant comes, though he does not. A new young soldier does the choosing, as to impress the Commandant.