Makes no sense to start downloading a separate widevine, current in chrome is 4.10.2710.0, so on the supported list.
To make it works with all browsers, it certainly can't be just in a qt6 directory. AFAIK, it still should be in the /usr/lib/chromium directory to be seen/used by most, might be needed to add an extra link for Falkon, but you can test that yourself too., just add the link.

    demm might be needed to add an extra link for Falkon

    Yes it works, just test it.

    I test with version in build, but symlink is incorrect it point out to:
    /opt/google/chrome-unstable/libwidevinecdm.so
    But it should point to:
    /opt/google/chrome-unstable/WidevineCdm/_platform_specific/linux_x64/libwidevinecdm.so
    /usr/lib/chromium/libwidevinecdm.so is incorrect too.