% !TeX TS-program = lualatex
\documentclass{article}
\usepackage{fontspec}
\usepackage{tikz}
\begin{document}
\newwrite\tempfile
\begin{tikzpicture}[remember picture, overlay]
\setmainfont{SauceCodeProMediumNerdFontComplete}
\node at ([xshift=12pt,yshift=30pt]current page.south west) {\symbol{"2615}};
\node at ([xshift=19pt,yshift=30pt]current page.south west) {a};
\immediate\openout\tempfile=szerokosc.txt
\newlength{\foo}
\settowidth{\foo}{\symbol{"2615}}
\immediate\write\tempfile{char 2615 \the\foo}
\settowidth{\foo}{a}
\immediate\write\tempfile{char a \the\foo}
\end{tikzpicture}
\immediate\closeout\tempfile

\end{document}


I get :
<szerokosc.txt>
char 2615 0.0pt
char a 0.0pt