What blew my mind is that it hasn't been proven that pi contains an infinte number of ones, for instance. It's not out of the question that there is a decimal place where the last 1 appears and there are none from then on.

It's not really likely, but we simply don't know and it is possible. It sounds weird given how many decimals of pi we've calculated, until you realise we've literally calculated 0% of them.

Whenever I think about the possibility of a Multiverse it just gets so unbelievably convoluted that I can't believe that that's how the Universe/Multiverse actually exists. Is the idea that every potential change in every atom or event in the Universe leads to all these other Universes, all co-existing, no matter how small & insignificant the differences? So we'd have a ridiculous number of Universes whose sole difference from ours is that a single atom behaved slightly differently in a rock out in the parking lot. Then multiply that by EVERY possible atom in the entire Universe, all behaving slightly differently.

That's just physical matter, what about conscious decisions made by living things? So in one Universe I filled my bowl of cereal with X oz of milk VS another universe where I filled it with X+1 oz of milk, and so on. All these micro-decisions that branch out into separate timelines, multiplied by the number of living entities in the Universe, every second of every day.

So are new Universes just constantly springing into existence at every moment in time, connected to every atom and every living thing, just brought about by tiny differences? I write some gobbledyremoved here: aksfhkashdf in one universe, adshfoasfdoajsidd in another, pooigjmasiodmfas in another, and so on. Multiple universes all suddenly springing into existence based on random key presses? Universes can't possibly be that "easy" to create can they, all that mass and energy, just poofed into existence, and it's constantly happening every second? Is mass, energy, and space just meaningless?

Or is it some other more basic set of differences describe the universe, just the starting conditions are different, but from there, each different Universe just proceeds as is, without multiple branching timelines? I'm not smart enough to understand any of it, it just quickly gets so incredibly convoluted and complicated for me to wrap my brain around.

CDs are optical storage, just plastic with tiny bumps. It's magnetic and solid state storage that can have bit flipping.

I think a lot of people assume a multiverse works that way because popular fiction makes it look like it does. However popular fiction is using something more akin to an omniverse (idk if there is an actual agreed scientific definition for a collection of multiple multiverses so Im just using that).

Using your analogy with the donkey kong discs being different universes with slight alterations in the warehouse (multiverse). In an omniverse scenario that you see in popular fiction, next door you'd have another warehouse but instead of donkey kong discs it is mario discs, or maybe donkey kong plushies.

However again that's all speculative of if there even is a multiverse let alone something larger than that

Natural numbers doesn't contain 2.5 because we define it so. Similarly all those CDs are practically the same because it's made in a factory designed to minimize the variance. Is there a similar strong will or intention in how a multiverse evolves?

It seems inefficient to run so many instances of the same scenario

You haven't disproved anything. The common understanding of multiverses typically only extends to livable multiverses, but there are infinite multiverses capable of sustaining logic and organization, just as there are infinite universes of junk data.

due to the nature of infinity — a la monkeys and typewriters — you could have not only a single CD that due to a catastrophic series of errors is actually something completely different from a CD — but an infinite number of them.

Is it entirely beyond the realms of possibility that an infinitesimally small stroke of luck could create a sentient race of CD people? Except "small" doesn't make sense in infinity — "small" just means "a less common certainty"

... but Australia doesn't exist???

Makes sense, for the universe with no multiverse theory to exist they have to be part of the multiverse since we exist in a universe where it does.

You are correct in thinking that there would be an infinite subset of universes where humans never evolved. However, there would also be a much smaller, but still infinite, subset of universes where humans did evolve. And there would be a muuuuuch smaller, but still infinite, subset of universes where the history of the universe and all of human history transpired exactly as it did in our universe, up until the point it diverged with the necessary changes in order to be relevant to the plot of the movie. That tiny tiny tiny subset is the only thing we talk about in movies, because otherwise it would be too confusing for general audiences. Hope this helps.

Such movies generally explain it as there are other universes locally clustered to yours. As in, there are so many universes that some are only a few decisions/variables off yours

would humans even evolve? Would life even evolve?

There could be different laws of physics in this universe that makes it near impossible for somebody outside of it to visit. Or, from another perspective, would you wilingly visit yourself?