Hi Coiln Park,
You need to set the QT_QPA_FONTDIR to the location of the font directory before launching the Qt application.
Please check out the below thread for more reference:
how to installing "segoe ui" fonts on to imx6qp yocto build
Also check Fonts section in Qt for Embedded Linux | Qt 5.14 .
Regards,
Karan Gajjar