\documentclass[12pt]{article}

\usepackage{fontspec}
\usepackage{polyglossia}

\usepackage{makeidx}
\makeindex

%\setdefaultlanguage{english}
%\setromanfont{Noto Serif}
%\setmainfont{Noto Serif}
%%\setsansfont{Noto Serif}
%\setsansfont{Noto Naskh Arabic}[Script=Arabic]
%\newfontfamily\arabicfont{Noto Naskh Arabic}[Script=Arabic]
%\setotherlanguage{arabic}

\setdefaultlanguage{arabic}
\setromanfont{Noto Serif}
%\setmainfont{Noto Serif}
\setsansfont{Noto Serif}
\setmainfont{Noto Naskh Arabic}[Script=Arabic]
%\setsansfont{Noto Naskh Arabic}[Script=Arabic]
%\newfontfamily\arabicfont{Noto Naskh Arabic}[Script=Arabic]
%\setotherlanguage{arabic}
\newfontfamily\englishfont{Noto Serif}
\setotherlanguage{english}

\pagestyle{empty}
\begin{document}
\thispagestyle{empty}

%\begin{Arabic}[numerals=arabic]%% "Arabic", not "arabic"
%\begin{Arabic}[numerals=mashriq]%% "Arabic", not "arabic"



زهرة\index{زهرة}
عصفور\index{عصفور}
ريح\index{ريح}
القمر\index{القمر}

%\end{Arabic}

\printindex

\end{document}

\index{}.