documentclass{article}
usepackage{nicematrix}

begin{document}
[
begin{pNiceArrayC}{CCC|C}
a_{11}  & cdots    & a_{1n}    & b_1       \
vdots  & ddots    & vdots    & vdots    \
a_{n1}  & cdots    & a_{nn}    & b_n       \
end{pNiceArrayC}
]
end{document}