I can invent a story that the coin in my pocket has two sides, each of which is in a superposition of heads and tails.
If you disprove that by repeatedly looking at both sides and noting that they always end up different, then I can invent the story that the observed side tells the unobserved side what to become.
If you disprove that by separating the sides by a great distance before observing them both, I can invent the story that they coordinate instantaneously, ignoring the speed of light.
This comment chain is a prime example of the ways quantum mechanics are (mis)understood and how many of us struggle to fully reason out the implications of each piece of it. I hope we can all learn something.
If you block one slit and then shoot particles, then block the other slit and shoot particles, you end up with a completely different pattern than if you have both open at once.
This isn't difficult to understand, the only difficult part is people not wanting to believe it is true and trying to come up with classical explanations for quantum mechanics, even though we have hard proof that classical physics can't explain this.
The difficult part is that even though the particle went through both slits, it only ever ends up in a single spot at the end, that is the "wave function collapse", it was a wave that went through both slits, and then it collapses to a single point when we measure it at the end. That is still not well understood by anyone today.
"Measurement" is a terrible word we've been using, along with "observer".
What matters is information propagating from one system to another or, perhaps better, when the superposition has any external effect, then the superposition collapses and the system assumes a classical state.
The key to understanding this is that _any_ interaction with anything counts as a "measurement" from the particles perspective.
I like to think that our simulation has a bandwidth compression algorithm that only decodes the state of a particle when it needs to calculate an interaction. Just like how your dungeon crawler doesn't spawn mobs until they are within interaction distance. But that's just fun head canon.