See: Matrices - Overleaf, Online LaTeX Editor

TypeLaTeX markupRenders as
Plain\begin{matrix} 1 & 2 & 3\\ a & b & c \end{matrix}
Parentheses\begin{pmatrix} 1 & 2 & 3\\ a & b & c \end{pmatrix}
Brackets\begin{bmatrix} 1 & 2 & 3\\ a & b & c \end{bmatrix}
Braces\begin{Bmatrix} 1 & 2 & 3\\ a & b & c \end{Bmatrix}
Pipes\begin{vmatrix} 1 & 2 & 3\\ a & b & c \end{vmatrix}
Double pipes\begin{Vmatrix} 1 & 2 & 3\\ a & b & c \end{Vmatrix}

Inline matrices

We can write small matrices with smallmatrix

LaTeXResult
\begin{pmatrix} a & b\\ c & d \end{pmatrix}
\big(\begin{smallmatrix} a & b\\ c & d\end{smallmatrix}\big)