Food tech is kinda my area, so I went and did a little research and it turned into quite a ride. For cultural context, grilled (broiled in the US, I think) cheese and Vegemite is kind of a traditional Aussie snack. Just a slice of white bread with butter and Vegemite, slice of cheese on top, stick it under the grill.

The Kraft singles I remember from my childhood absolutely did not behave like anything resembling real cheese when you did this. It melted on the inside, sure. But the outside just dried out and turned into a kind of plasticky skin, then bubbled and burned. So you were left with this partially blackened and crunchy cling-film like skin disguising a thin layer of vaguely dairy-adjacent molten plastic goop that was guaranteed to stick to and sear the roof of your mouth, while the skin came off in one piece and slapped you on the chin with the equally hot residue of said plastic goop. For some reason kids loved this.

I’m not sure when OP last ate them, but the Kraft singles I know got axed in like 2017 when Mondelez sold their cheese line to Bega. That makes it incredibly hard to track down the original formula to figure out what in the world they were really made of. They have, however, since been re-released and claim to be at least 45% cheese, which I suspect is a lot more than the ones I remember, probably does melt, and falls pretty squarely into the “processed cheese” definition according to FSANZ. There’s no way in hell I’m buying some to try it though.