That’s mostly a Linux (graphic server) thing. Oversimplifying things a lot (I hope Linux gurus do not get too mad about this), in Linux you have (at least) two, let’s call them “flags” you can put on selected text. The primary flag applies when you select any text, but only when you press the middle mouse button that text gets copied and then pasted to the mouse pointer position. The other flag is what’s normally associated with “the clipboard” and is acted when a particular shortcut is applied:
C “claims” the selected text to that second flag while
V paste the claimed text into the target position.
There is a lot more nuance to all this, of course, but don’t worry: most Linux desktop environments come with their own clipboard manager utility. In fact, you can make Klipper (KDE utility) ignore selections.
About using a middle button without the middle button: generally a middle button action is simulated by clicking the left and right button simultaneously.