They're large piece of plastic filled with data. Uh, they're usually in different shades of gray, but there are ones in black, red, or even ones with clear casing. It depends on the company who makes 'em. And they also have the names of the games on them for easy identification.

[ he leads them around the battery aisle, turning left. ]

But wait, you've gone to a different world before this? [ at least that's itaru's understanding of it??? more isekai adventures....... ]