Think of the universe as a painting. There’s the image made in paint, and the surface it was painted on. The canvas.
The stars, the planets, the gasses, the matter and energy and even the space between are the paint. What’s the canvas? Is there a canvas? Would the canvas follow the same rules as the paint?
Excellent comment, love the diagrams.
But we know spacetime is getting bigger, because the universe is expanding. So what is spacetime expanding into?
Who knows? Are we in a bubble floating in a higher dimensional sea of different bubbles? Is it nothing, like a balloon expanding in the vacuum of space?