oldschool ( from terminal-only era ) => copy by selecting text (mostly in terminals) and paste using middle mouse click
more modern new style ( based on graphical interfaces ) => copy by ctrl+c (or ctrl+shift+c in terminals and such) and paste ctrl+v (ctrl+shift+v in terminals …)
Normally both those clipboards work independent as they are handled by 2 different processes, so you can for example copy one text using ctrl+c and copy another text by only selecting second text, then you can paste both, one with middle click, second with ctrl+v
More and more distro have a clipboard managers that have a feature to “sync” both clipboards, but it’s a lot of time disabled because it’s more confusing people and sometimes annoying. Why it’s confusing and disabled by default? Imagine that you selected some text, then did a ctrl+c, you move to some word document, select text, remove it, and want to Paste it. Guess what, the selecting you did to remove text did copy the selection to clipboard overriding what you did have there from ctrl+c.
Most if not all terminal emulators (konsole, gnome-terminal, xterm) support both clipboard styles, old-school select to copy, and new one but because the “ctrl+c” shortcut reserved to stop/interrupt applications they all decide to use ctrl+shift+c to do a copy. And yes, not only terminal emulators use ctrl+shift+c, I did have few encounters on some random apps, but most of the time, if ctrl+c is used for something else, ctrl+shift+c was available.