"Okay, so picture a room. There's a control panel with a ton of dials and a big red button, and - sure - a cute poster of a kitten and some stale coffee in a mug and whatever else would be in a control room. The dials are different sizes because some of them have different numbers of settings, and each one is for some basic law of physics. There's more dials than you might think, because some of them are set to 0 in your universe so it would be hard for you to even figure out they exist.
"So now imagine that someone has the job of sitting at this desk and adjusting dials, and then pressing the button to send the settings to the production floor. He's on duty until he has churned out every possible combination of settings, which is some absurdly large number that I don't have time to calculate. It's big enough that describing it as big isn't really sufficient. He doesn't get overtime for this.
"When the production floor gets a full set, one of each combination, they make each one into a universe. This all happens at the same time but for the sake of this example let's pretend they churn them out on a conveyor belt. So there are some, actually quite a lot, that sort of fizzle and collapse on themselves instantly. That's fine, that's expected, nobody freaks out. Then there are some that are more stable but still not self-sustaining, and those get some sort of red mark from the quality control guy as they slide past on the conveyor, shrinking or crumbling or whatever it is they do.
"If the person at the control panel could see this he would cry, he would threaten to quit out of frustration because it would be obvious that a huge number of the failures were predictable. He could watch for a bit and then draw some lines next to some of the dials with a permanent marker because any universe, any at all, with a setting of more than N on this dial will fail. There are other rules that are more complicated but still pretty observable, and he could make a technical manual talking about all this but he won't because it wouldn't change anything. They'll make all of the universes every time.
"But despite this wasteful behavior, they get a lot of useful universes. There are so many, even with all the failures, that it would still be a stupidly large number. Many of them can't support life as you know it which might make you think that they're pointless but that's only because you have a sadly limited view on the business end of the whole multiverse thing. Others have plenty of life, and one of them is your universe. That's obvious, of course yours is in there somewhere - but what isn't immediately obvious and causes some people a lot of distress is that there are other universes nearby with such a tiny difference in settings that you're in there, too.
"This has to be a small difference. Really small. In most cases, even the smallest difference there is would keep you from being born because over time, from the conception of the universe to the present, that difference is magnified. Picture our control room guy getting bored and removing two dials, nearly the same size. Roll them once on the floor and they both end up in the same basic place. Roll them ten times and you may see some difference. Roll them a thousand times and they're pretty far apart. It's like that.
"But that's partly a function of time, and guess what one of the dials is for? What I'm saying here, is that the smallest unit of time is still a discrete unit, and if you change the size of that unit you get universes aging at different rates but with the same events taking place. I could talk here about quantum stuff, things being truly random, but the short version is that they're not random they're just seriously complicated and when you think you see something that's in an unresolved quantum state there's still no actual chance involved in which way it resolves.
"So you could be out there, somewhere, although not an infinite number of you because as I said even the smallest difference grows, like the space between dials as they roll across the floor. If your quantum of time is 1, and mine is 2, and the next guy's is 3 - these being incredibly small differences, you understand - then after one objective external unit of measurement the times are 1, 2, and 3. But next they're 2, 4, and 6. Then 3, 6, and 9. The gaps get bigger all the time, and after billions of years when you come on the scene you can imagine that there aren't going to be a lot of other universes that currently have a you living there.
"Back to the factory - and I haven't forgotten that beer, I'm picking up the tab so just stay with me - so, right, these universes don't get made just the once; they get churned out every time the guy in the control room completes a set. If your universe gets made a hundred times, since there's no random stuff going on in it you'll be born and live and die the same way in each of them. But this example, this is where it would seem to fall apart. Because in a factory mistakes get made. Someone drops a screwdriver into the gears, the machine needs to be oiled, a universe gets dropped off of the conveyor, whatever. Can that happen here? If so, we're talking about some sort of meta-universe with the other universes in it, and that leads to the question of whether or not the factory was made in a factory, and where that meta-factory was made, and so on.
"If there's no meta-universe it gets a bit foggy talking about relative ages and locations of the universes, and the concept of more of them existing in the same way... it's kinda messy. Fortunately, there really is a factory. Like, actually, a literal factory. And it doesn't need a meta-factory because it's on a complicated sort of self-causing loop, which doesn't violate causality because of the rules of the reality it exists in. It's not a satisfying answer, but there you go.
"And - no, hang on, we're almost there - you worked there, or someone that was essentially you. And you've recently had a bit of an accident and been sort of pulled into the machinery, that being in this case an impossibly small space containing enough raw materials for a few decent-sized universes, and as such your position is now open. And here you are, one of only twelve of yourself available and one of only three that are the ideal age and frame of mind and whatnot, and what I'm asking is if you would like a job. Yes, yes I'm serious.
"Oh, come back. Please? Look, a nice fresh beer just arrived! Come on, I'm going to be in a lot of trouble if you're not there when they realize you're not there.