I don’t understand this well myself, but my understanding is that the 3D universe wraps into a ball in the 4th dimension. Just like on a balloon the 2d space on its surface wraps in the 3rd dimension. To creatures who can only perceive a 2d world it seems odd that their world has no limit, but from a 2d perspective you can’t reach some limit on the surface of the balloon. Just as you cannot travel in one direction on the earth’s surface and ever reach a limit.
The size of our universe would then the be circumference of this ball. But that doesn’t mean there is a world outside this ball.