\documentclass{article}

\usepackage{fontspec}

\setmainfont{SourceSerif4Variable-Roman.otf}

\begin{document}

\addfontfeatures{RawFeature={+axis={wght=100}}}

The quick brown fox jumps over the lazy dog. \par

\addfontfeatures{RawFeature={+axis={wght=300}}}

The quick brown fox jumps over the lazy dog.\par

\addfontfeatures{RawFeature={+axis={wght=300,opsz=10}}}

The quick brown fox jumps over the lazy dog.\par

\addfontfeatures{RawFeature={+axis={wght=300,opsz=24}}}

The quick brown fox jumps over the lazy dog.\par

\addfontfeatures{RawFeature={+axis={wght=600}}}

The quick brown fox jumps over the lazy dog.\par

\addfontfeatures{RawFeature={+axis={wght=900}}}

The quick brown fox jumps over the lazy dog.

\end{document}