The Nine Realms were connected. Yggdrasil ran through all nine of them and was divided into three layers. Its power and branches were like invisible star routes, linking every place through unique force fields, as though strings had been tied to nine balloons in the universe.
Loki knew many passages from Asgard to other planes. They could be understood as wormholes in the trunk: drill in from this side and slide out the other.
Helheim was one of the more unusual ones. Located beneath Yggdrasil's void-like roots, it was the only plane among the Nine Realms not directly connected to the Main Dimension. Strictly speaking, it was also a Hell.
Yggdrasil was, after all, an Elder God of Oshtur's generation. Though it had taken another path and had not become one of the Dimensional Demon Gods, it had formed a unique self-sustaining system. That was a plant's instinct.
The Aesir, the Vanir, giants, dwarves, elves, and the other creatures living in the Nine Realms all had dual identities, except for humans—they were both its leaves and its fertilizer.
What it gave them was destined to be reclaimed one day.
Humans were the exception, because Earth had not been created by Yggdrasil, nor were humans its creations. Their souls were "recyclable energy" coveted by countless beings.
If those races were leaves, then humans were fruit. The difference was that fallen leaves rotted away and became nutrients for Yggdrasil's rebirth after winter, while fruit was picked away batch after batch by others, harvested at regular intervals.
When people died and their souls left their bodies, it meant the fruit was ripe. Those fruits even rolled into their corresponding Hells on their own, practically like an automated assembly line.
And so-called winter was Ragnarök. The cycle was the flourishing and withering of plants.