Qt has a feature of Custom Widgets in Qt Designer (https://doc.qt.io/qtforpython-6/tutorials/basictutorial/uifiles.html#custom-widgets-in-qt-designer)
But it doesn't work when python plugin-file is put in PYSIDE_DESIGNER_PLUGINS directory.
I.e. I have a project that works completely fine under Windows OS - Qt Designer shows customer plugings.
When I open Qt Designer under Arch Linux using the same plugin directory - there are no custom widgets present in GUI.

As "Troubleshooting the Qt Designer Plugin" section says (see above link) I went to Help/About Plugins menu and there is an error indeed:
Cannot load library /usr/lib/qt6/plugins/designer/libPySidePlugin.so: (/usr/lib/qt6/plugins/designer/libPySidePlugin.so: undefined symbol: PyImport_AddModule)

The library itself is present in my filesystem:
[user@pc designer]$ file /usr/lib/qt6/plugins/designer/libPySidePlugin.so
/usr/lib/qt6/plugins/designer/libPySidePlugin.so: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=815fecdaf41fbf92f72cef2f48981edcdc1b2ecb, stripped

I suspect that the feature doesn't work because this part of manual was missed when Arch-package was compiled:
When building Qt for Python, be sure to set the --standalone option for the plugin to be properly installed.