I am making an assumption that you grew up with something like an iPad. This isn't your fault, you had no reason to learn it before. Now that you have a need to do these things, you figured it out by accident just through exposure.
KDE
KDE is an international technology team creating user-friendly free and open source software for desktop and portable computing. KDE’s software runs on GNU/Linux, BSD and other operating systems, including Windows.
Plasma 6 Bugs
If you encounter a bug, proceed to https://bugs.kde.org, check whether it has been reported.
If it hasn't, report it yourself.
PLEASE THINK CAREFULLY BEFORE POSTING HERE.
Developers do not look for reports on social media, so they will not see it and all it does is clutter up the feed.
ipad
Haha. Well, my first computer was an Apple II, if that counts...
Are you talking about selecting multiple unconnected sections of text, so that they are highlighted at the same time? I think that's a Firefox feature.
Or are you talking about selecting something and then something else, so that only the last thing is highlighted, and finding both selections listed as separate items in KDE's clipboard manager?
Talking about the first. Seems to work in Libreoffice as well, but not Kate and Konsole. So it's definitely app specific, not a KDE thing.
Firefox and LibreOffice aren't part of KDE, but Kate and Okular are. So the fact only the former two support this feature indicates it's not from KDE. This should probably be supported by KDE though (or by Qt? perhaps this should be done on the toolkit level rather than on the application level). Holding ctrl to multi-select is standard behaviour in a lot of applications and is quite useful. For what it's worth, Krita does support this, but you need to hold shift instead of ctrl.