documentclass[a4paper,11pt]{article}

usepackage[tuenc]{fontspec}
usepackage[a4paper]{geometry}
usepackage[french]{babel}

usepackage[bold-style=ISO]{unicode-math} %works without this
    setmathfont{Latin Modern Math}
    setmathfont[range={mathscr,mathbfscr}]{XITS Math}

begin{document}

$x+y=z$

{boldmath$x+y=z$}

$mathbf{x+y=z}$

end{document}