Nowhere. The universe's expansion is the 3D equivalent of the surface of a balloon that's getting filled with air. It's not a sphere that's getting bigger either, you could say space is "growing" from everywhere at the same time, it's why the expansion of the universe is making galaxies be pulled apart from one another.

Thinking that way, which isn't wrong, means there must be an edge somewhere. What would that be like? An invisible wall u just can't move past?

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The universe isn’t expanding into anything since it is everything. It’s the space in the universe that’s getting bigger. That doesn’t make intuitive sense because it not something that we experience at the size of things we interact with.

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It expands into nothing.

We have to assume that prior to the universe there was nothing. Literally nothing. Not even a big, open space. Just nothing. It's hard to wrap your head around "nothing". We think of it like an empty box. Thing is, the box doesn't exist and neither does the vacant space within it.

That's why the universe will expand infinitely. It's headed outward into something that doesn't exist.

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Even if you follow your logic and our universe was expanding into another volume, you can ask the same question of that — where did it come from, ad infinum.

But even worse, that would all be speculation because anything “outside” our universe is inaccessible to observation anyway. By definition our universe is all matter, energy, space and time contained therein. There is no “outside”.

Your question has been asked many times with regards to the balloon analogy. “But where is the balloon a inflating into?” In the analogy, that’s higher dimensions. But that’s the rub — it’s just an analogy and not literal. Space “is” and expands in a measurable way. Always. Using an analogy (like the balloon) and concluding “must”, well, that’s a fallacy. The analogy helps describe some aspects of what we observe, but it’s not reality.

As for the Big Bang, all energy was contained in a singularity that was a state beyond which our physics can describe. Time, space, forces, and energy were all unified. That’s why there was no “before” the Big Bang because just like there was no space, there was no time. All of our dimensions, space and time, were created with the Big Bang.

One last note, quantum field fluctuations and uncertainty are natural part of or laws of physics. It’s real and tested. “What triggered” the Big Bang may simply be random quantum wave fluctuations in whatever state the universe was in. No special action was necessary. Our current understanding of physics simply isn’t advanced enough to describe that state. Not without quantum descriptions for gravity and maybe time too.