All of this is confusing af

At least in Germany, at a lot of Rewes (supermarket chain), this is absolutely a thing and very common. You can place the handheld barcode scanners in a specialised holder on the cart handle and then scan as you go, and neatly package all your stuff before going to the checkout and paying at a terminal. If even Germany has got this by now, then every other country on the planet surely does too lol.