Very basic example. fontspec is very powerful.

MWE
\documentclass{article}
\usepackage{xcolor}
\usepackage{fontspec}
\setmainfont{Noto Serif}
\setsansfont{Noto Sans}
\setmonofont{Noto Sans Mono}
\newfontfamily{\fcyc}{cyklop}
\newfontfamily{\falg}{AlgolRevived}[Scale=1.5,Color=red]
\newfontfamily{\fsong}{FandolSong}
\begin{document}
\Large
This is the default roman text for the document (the main font).
{\sffamily This is the sans font. 123456}
{\ttfamily This is the mono-spaced font: 123 456 789.}
Here is an example of a font-switch: \falg ABC abc 123 \normalfont and then switch back to normal font.
{\fcyc Abc cyklop font} -- this font is restricted to a group with { }.
Some random characters in FandolSong font: \fsong 一丢人伺\normalfont
Back to normal font again.
\end{document}
fontspecpackage and, if so, have come across the\setmainfont,\setsansfont,\setmonofont, and\newfontfamilycommands. (Hint: to open the user guide in a pdf browser, open a command window and typetexdoc fontspec.) – Mico Apr 16 '21 at 05:25\Ofontis a switch. – Cicada Apr 16 '21 at 05:45\newcommand{\myofont}[1]{{\Ofont #1}}and then do\myofont{Some text printed in Oswald Regular}. Note the extra brackets to restrict the font switch to within a group. – Cicada Apr 16 '21 at 05:48