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.
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.