When we say that space "expands" you probably imagine in your head watching something that grows in front of you. For example, you inflate a balloon and watch it expand, and you imagine that what is inside this balloon represents the universe that is expanding into the ambiant space where you are.

This is the wrong way to look at things, because it relies on that notion of "ambiant space"

Instead, imagine that you are Pac-Man, living in the Pacman world. For the sake of the example, let's say that the Pac-Man screen is 10m wide. For you the the world is finite and is exactly 100m^2. But more importantly, if you walk straight in the horizontal direction, you will get back to where you started after 10m. Same thing if you walk straight in the vertical direction (that's the main feature of the PacMan world). For Pac Man, there is absolutely nothing outside this 100m^2 of land. There is no "ambiant space" where this 100m^2 land sits in. There are no "exterior wall" walls in the Pac-Man universe.

Now, imagine that by some mechanism, the ground of the universe is getting bigger. The mechanism itself is not important, but you can imagine that the ground of the Pac-Man world is made of metal, and that you are applying heat to that metal (and metal expands when it's getting hotter). The metal will expand a little bit, by 10% in each direction for example. Now the world, is no longer 100m^2 but is 121m^2 . Now you have to walk 11m in the horizontal direction to get back where you started.

And yet, the Pac-Man universe didn't expand into an already existing piece of land. As there are no "borders" to th Pac Man universe, you cannot consider that the borders have moved further. It's just the ground that you are walking on that gets bigger.

This is the picture that you should keep in mind when imagining the universe expanding. Not as an outside observer, but as someone inside. "Expanding" just means that there is more space to cover to get from point A to point B.