>>105839153a backup, put simply, is a second copy of important data. that's all it is. with a strong emphasis on said copy being a cold copy, meaning it's not physically attached to a computer except when being updated
there's countless ways to do this, even just the most basic form which is taking an external hdd and coping data to it and storing it is fine for most people, the important thing to remember is that it is a COPY. just having data on an external drive is not a backup, because the defining property is not whether it's external, but whether there's more than one copy
put simply, if you could run a script that deletes everything currently attached to your computer, internal drives, external drives, and even network drives. anything you can't restore is not backed up. period