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.

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?

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.