nfss-declare-math-symbol.tex

\documentclass{ctexart}

\DeclareSymbolFont{cnmath}{TU}{SongtiSC(0)}{m}{n}
\DeclareMathSymbol{,}{\mathpunct}{cnmath}{"2C}

\input code-with-output

\begin{document}

\setcounter{tcb@cnt@example}{2}
\begin{example}{(Re)declare math symbol}
  % \DeclareSymbolFont{cnmath}{TU}{SongtiSC(0)}{m}{n}
  % \DeclareMathSymbol{,}{\mathpunct}{cnmath}{"2C}
  
  \LARGE
  text mode: english, comma 中文，逗号\par
  math mode: $abc, abc$
\end{example}

\begin{example*}{Print current font info}
  % ref: https://tex.stackexchange.com/a/303536/79060
  \newcommand{\printInternalCurrentFont}{%
    \expandafter\ttfamily
    \expandafter{\expandafter\string\the\font}%
  }
  
  中文\printInternalCurrentFont
\end{example*}

\end{document}