\documentclass{article}
\usepackage[a4paper]{geometry}
\usepackage[ngerman]{babel}
\usepackage[autostyle]{csquotes}
\newcommand\demo{Some text in the default font style IBM Plex Serif }
\pagestyle{empty}
%StartVisiblePreamble
\usepackage[DefaultFeatures={Scale=0.94}]{plex-otf}
%StopVisiblePreamble
\begin{document}
\demo\ Regular\par {\PlexTextRM\demo\ Text\par} {\PlexLightRM\demo\ Light\par}
{\PlexExtraLightRM\demo\ ExtraLight\par} {\PlexThinRM\demo\ Thin}
\end{document}