If it’s expanding, what is it expanding into?
I think you should start with:
https://en.wikipedia.org/wiki/Observable_universe
And only then with:
https://en.wikipedia.org/wiki/Shape_of_the_universe
In short, what can ever influence "our" part of the universe is limited by the speed of light being constant and being the physical "speed limit", and by the fact that the age of the universe is less than 14 billion years. (Earth alone is 4.5 billion years old).
Fascinatingly enough, we can "see" in each direction we look at up to the distance of around 46 billion light years, even if the age of the universe is just 13.8 billion years (that 46bly in one direction is the result of the accumulated expansion of the universe since the beginning). And the farthest things we see are also the oldest, as we effectively "look back in the past" when we look at the distance (due to the speed of light being constant). So when we look far enough we see almost up to the beginning of the universe (up to the point when the universe was "clear enough" for us to see after the beginning). The CMB (Cosmic Microwave Background) is the "leftover glow" of the big bang that reaches us now, from that earliest past:
https://medium.com/starts-with-a-bang/this-is-how-we-know-th...
Then one can try to guess what's there behind the physical limits that are fixed for us, but we will never have the direct data about that:
https://medium.com/starts-with-a-bang/ask-ethan-how-large-is...
Maybe akin so asking: "What came before time?". The question itself is a contradiction.
Infinite Universe
https://www.amazon.com/Infinite-Universe-Theory-Glenn-Borcha...