You can use \fontname\font
in the LaTeX document to see what font is used by fontspec
. For example:
\documentclass {article}
\usepackage{fontspec}
\setmainfont{CormorantGaramond-Regular.ttf}
\begin{document}
\scshape \fontname\font, Hello World
\end{document}
You can see in this example:
"[CormorantGaramond-Regular.ttf]/OT:script=latn;language=dflt;+smcp;mapping=tex-text"
So, your plain TeX document can look like:
\font\f="[CormorantGaramond-Regular.ttf]/OT:script=latn;language=dflt;+smcp;mapping=tex-text"
\f Hello world
\bye
Note that the font feature +smcp
does this folk: it switches the font to the Caps and Small caps mode.