The short video can be summarized in one simple sentence.
IF qt5ct is installed, change your icons in the QT5 Configuration Tool.
Take a look at the top image.
Long and technical version
In the long video we will show you the code inside the configuration files and reboot many times in order to see and understand where the settings are coming from and why things seem not work.
You will see why icon themes ARE NOT APPLIED even when you told Plasma to apply it.
QT5CT is installed
the settings from ~/config/qt5ct/qt5ct.conf are applied.
Change the settings via qt5ct and apply.
Qt5ct overrules the settings of plasma.
QT5CT is removed or not installed
the settings come from ~/.config/kdeglobals
Change your icons in system settings now.
Over time we will evaluate the best approach for plasma.
To qt5ct or not to qt5ct, that is the question.