What is $z_b$ in this introduction to Private Information Retrieval?
Long to write as a comment;
Look at the referenced paper 47

The user chooses uniformly at random $t$ numbers $y_1,\ldots,y_t \in Z_n^{+}$
such that $y_b$ is a $\text{QNR}$ and $y_j$ ,
for $j \neq b$, is a $\text{QR}$. It sends these $t$ numbers to
DB (total of $t \cdot k$ bits).

The database, DB, computes for every row $r$ a number $z_r \in Z_N^*$ as follows: It  first computes (in $Z_N^*$)
$$w_{i,j} =  \begin{cases} y_j^2 &amp; \textbf{if } M_{r,j} =0\\
y_j &amp; \textbf{if } M_{r,j} =1 
\end{cases}$$
and then it computes
$$z_r \ \prod_{j=1}^{t} w_{r,j}$$


...
Therefore, one needs to look at the references to see them in more detail. Survey authors may skip some steps that may trivial to themself but that is not true for all.