One time someone asked this question and someone gave the example of a computer screen and it totally clicked for me. Imagine the universe is a computer screen. When we say the universe is expanding we’re not saying the screen is getting bigger. It’s not going from a 30 inch to a 40 inch. The universe is already infinite it can’t get bigger. We’re saying the resolution is getting better. A few billion years ago it was standard definition. Two pixels on opposite sides were a few hundred pixels apart. Today though it’s 4K. Thousands of pixels apart. They didn’t actually move into new areas but now there’s more space between them. Get it?