I've seen this document before, but it's inaccurate as it doesn't exist in /opt/Qt/6.5.0/gcc_64/bin/plugins/designer. Instead, it exists in /opt/Qt/6.5.0/gcc_64/plugins/designer/
I put my .so plugin in the folder but in Qt Designer going from the menu: Tools > Form Editor... > About Qt Designer Plugins the copied plugin is not displayed, even by refreshing.
Added after 1 14 minutes:
I apologize
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