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 gobbledygook 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.

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?

... 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.

would humans even evolve? Would life even evolve?

One small change can lead to a totally divergent path. Think about it this way. You are supposed to meet your SO today at work. But you forgot to brush. No big deal. Right? Well your SO says hello, so you say it back and then she gets a whiff. Well end of conversation and that’s that. No wife. No kids. Not with her.

Or maybe you miss that buss that you barely caught. Or maybe you win the lotto at 19 and that changes everything for you.

You’re thinking wrong if you think lives have to be drastically different in every way for it to manifest as a whole new life. It can be the smallest thing but that tiny, infinitesimal thing can lead to a cascade of change or shunt people down an entirely different path.

Lastly, your ancestors would also be subject to these events, making all their lives very different as well.

Butterfly Effect.

Or the laws of physics are just the same between all of the multiverses, and it's impossible to travel between them. Maybe the walls between universes are so thick that nobody will ever even detect that the other universes are there at all, making it basically the same as there being no other universes in the first place

Well, no, for a few reasons:

We don't know that, if multiverses are real, that it is possible to travel between them. Infinite variations doesn't guarantee this, because that just effectively means that all possible universes exist. Were multiverse travel to turn out to be impossible, then nowhere in an infinite multiverse would it exist.

Smaller and larger infinities exist. If the set of universes where you exist is larger than the set of universes where you have multiverse travel, then most of the time, you won't be in a universe that gets you a visit, even though the number of both is infinite.

If the way the system works is that a new universe is created for each possibly each time something happens, then every time someone travels to a different universe, it will diverge into one where the visitor arrives, and one representing the option that the visitor does not arrive. No matter how much travelling is done, there will still be a set, indeed an infinite one, representing the scenario where no visitor arrives is what happens every single time any multiverse travel occurs. In that set of universes, you never have received any visits.