The libraries were generated on debian 11 with gcc 10.2.1-6, qt 6.3.1 and qt 5.15.2.
Put the libfcitx5platforminputcontextplugin.so
to the path below:
<QT_INSTALL_PATH>/gcc_64/plugins/platforminputcontexts/
And change the permmison:
chmod 755 libfcitx5platforminputcontextplugin.so
Then you can enter Chinese characters with fcitx in your qt application now.