Which volume is to be 'ejected'?

New 2020 Mac 27". Arrived with Catalina and immediately asked for Big Sur to be installed. Obeyed. Now, no documents will open - zip - nil - none: .pdf, word, excel, they opened in Cataliana but not Big Sur. Each document gets an error 'unrecognised type or damaged'. So now I have to wipe and install Big Sur afresh according to Apple Support online. So to this page I am (mis)led and; ------

So under 'Use the 'createinstallmedia' command in Terminal' point 4. speaks of a 'volume'. Point 5. also speaks of a 'volume' and goes on to instruct the user to 'eject the volume.' Which 'volume' then, is to be ejected? Bit confusing to the uneducated among us. Me.