Thanks to @HenriMenke, I know how to use Roboto font in math for letters (including Greek) and digits. Now, I'd like to use a sans serif font for math operators as well (say \int or \sum). AFAIK, Roboto does not provide such glyphs.
How can I do (I mostly use lualatex but I can use xelatex if necessary)?
You could use e.g. Fira Math:
\documentclass[12pt]{article}
\usepackage[sfdefault]{roboto}
\usepackage{amsmath}
\usepackage{unicode-math}
\setmathfont{Fira Math}
\setmathfont[range=up]{Roboto}
\setmathfont[range=it]{Roboto-Italic}
\setmathfont[range=\int]{Fira Math}
\begin{document}
Hello world! 12345 $12345$
\textit{abcdef} $abcdef$
$\gamma = G$
\[\int \sum a^2 + b^2 = c^2 \]
\end{document}
