Definitions of Commands and Environments #

\newcommand<new command (with \)>(number of arguments)[default value for the first argument]{specific command definition}

\documentclass[]{article}
\usepackage[heading=true]{ctex} % Be sure to set heading to true, otherwise ctexset cannot be used
\usepackage[left = 30mm,right = 30mm]{geometry}
\newCJKfontfamily\qingsong{FZQKBYSJW--GB1-0}
\newcommand\jinse{\qingsong 锦瑟}
\newcommand\js[3][锦瑟]{#3#1#2}
\begin{document}
\jinse \jinse \par
\js{无端}{五十弦}
\end{document}

\renewcommand<existing command>(number of arguments)[default value for the first argument]{specific command definition}
The maximum number of arguments is 9, and the parameters are represented as #1~#9 in the command definition.

\newenvironment{name of environment}(number of arguments)[default value for the first argument]{content before environment}{content after environment}
The content in "content before environment" will be processed before the text contained in this environment, while the content in "content after environment" will be processed when the \end{name} command is encountered.
The maximum number of arguments is 9, and the parameters are represented as #1~#9 in the command definition.

% When using default argument values
\newenvironment{shengming}[1][Declaration]{
	\begin{center}#1\end{center}
	\begin{center}
}{\end{center}}
------
\begin{shengming}
锦瑟无端五十弦，一弦一柱思华年
\end{shengming}

% When using custom argument values
\setCJKfamilyfont{qingsong}{FZQKBYSJW--GB1-0}
------
\newenvironment{shengming}[1]{
	\begin{center}#1\end{center}
	\begin{center}
}{\end{center}}
------
\CJKfamily{qingsong}
\begin{shengming}{I declare}
锦瑟无端五十弦，一弦一柱思华年
\end{shengming}

You cannot reference arguments in the "after" content, so you need to find a way to do it in the "before" content.
\newenvironment{zhuyi}[1][Attention]{
	\newcommand\zy{#1}
	\textbf{\zy}\hfill
}{\hfill \textbf{\zy}}
------
\begin{zhuyi}
锦瑟无端五十弦，一弦一柱思华年
\end{zhuyi}