I watched a bunch of fan theory videos after it ended and the one that made the most sense to me was something like this:
The giant’s directions at the beginning were directions into an alternate dimension where they could trap that soul-eater thing that feasted on human misery. Taking Laura to that place at the very end allowed the soul eater thing to finally find her again, which is when she screamed and the show ended. This video theorized that when she remembered everything and felt all that pain, it overloaded the soul eater and killed it. Bringing Laura Dern to that other dimension and going through that painful and traumatic love scene also played a role in getting the soul eater to enter that dimension after them.
It’s been long enough that I can’t remember a bunch of the details, and I’m quite certain my breakdown of that fan video makes no sense. Oh well. It was a fun trip to watch.