Memes

You're still missing the 2D to 3D translation...

Think in two dimensions. The surface of the balloon is the two dimensions - X and Y axes. The "medium" (the rubber of the balloon) is the fabric of the universe.

The two dimensional space - much like e.g. the surface of the Earth - has no edges. When the balloon expands (on the third dimension, which the surface dwellers are unaware of), all the surface notices is that any two or three or N points have gotten further from each other. The size of this 2D plane thus can be described by a single dimensional number - the circumference of the balloon (aka "how long does it take to get back to the starting point in the 2D space if you go in any direction).

You need to extrapolate this continuous 2D surface on a 3D object to a 3D surface on a 4D object. In a four dimensional space, our universe has a four dimensional volume it consumes - but in 3D, it's effectively infinite (curving into itself, much like the surface of the balloon), but also expanding (everything is being further and further from each other).

To grasp this I'd recommend you read the book Flatland by Edwin Abbott - it's a 140 year old study in the format of a novel, on how beings from an N spatial dimension plane would "see" and interpret a being from an N+1 spatial dimension (in this case, 2D beings, like a square, seeing 3D being, a sphere, intersect with its 2D plane). Mind you it's also a social satire so you'll find a ton of social commentary of the Victorian era Britain, but the core conceptualisation is there - an N spatial dimension being will always have trouble visualising an N+1 dimension.

So again, in our 3 dimensional plane, the universe has no edges, it's infinite, yet expanding. Just like how the surface of a balloon or bubble has no edges, or how the surface of the Earth has no edges - but in a 3D space you can create a bounding box to define its size. Our 3 dimensional universe is essentially the surface of such a 4D bubble, but that fourth dimension is outside of our universe therefore it can't easily be measured directly. Just like how a dot on the surface of the balloon couldn't measure the 3 dimensional space of the balloon, only by inference of the expansion, theorise that it actually exists in a 3 dimensional system but is limited to the 2D surface of said balloon, and infer its mechanics based on the expansion of space between any two points on the surface.