In menu Bookmarks > Organize Bookmarks > (a window gets opened with the title "Library" -> shouldn't this window name be "Bookmark Library"?) existing bookmarks and folders cannot get cut, copied nor paste somewhere else in this window using the context menu of the mouse. So you cannot properly sort your bookmarks and folders there. SOFTWARE/OS VERSIONS KDE Plasma: 5.27.5 KDE Frameworks Version: 5.103.0 Qt Version: 5.15.8
Confirmed, there is no explicit Copy&Paste option. The workaround is to use mouse and Drag&Drop the entries to given folders. (it only moves, no copy here as well)
I can reproduce the behavior described in this bug report in Falkon version 24.08.1 which is built for KDE Plasma 6.