Pullback Diagram

\usepackage{tikz-cd}

\begin{tikzcd}
  P \arrow[r, "p_1"] \arrow[d, "p_2"'] \arrow[dr, phantom, "\lrcorner", very near start] & X \arrow[d, "f"] \\
  Y \arrow[r, "g"'] & Z
\end{tikzcd}