In some cases, when you open a file to modify it, the program (an editor of some sort) opens a new file with the data copied from the old file. That’s what you’re altering. When you’re done, the old file is still there as a backup. (Any earlier backup is deleted first.)

If you alter the file in place (many editors do that), if you have the file on an HDD, the original file can usually be recovered, but at an exorbitant price (in the thousands of dollars). It requires a clean lab and a scanning electron microscope.

It’s so much easier and cheaper to just make a copy of the file before you change anything.