It's probably the matte oleophobic coating that's being scratched. The iPhone 11 and up started having this coating on the back glass. It naturally wears away with use anyway, especially around the corners being taken in and out of a case, or if no case it will wear off over time.

You can try and protect it I guess, but I wouldn't bother. There are products available that can reapply a similar coating, or you can also polish it off to just a glass surface.