https://en.wikipedia.org/wiki/Norse_cosmologyThe root in the Æsir homeworld taps the sacred wellspring of fate, the Well of Urðr. The tree is tended by the Norns, who live near it. Each day, they water it with pure water and whiten it with clay from the spring to preserve it. The water falls down to the earth as dew.
...
Animals continually feed on the tree, threatening it, but its vitality persists evergreen as it heals and nourishes the vibrant aggression of life.[1] On the topmost branch of the tree sits an eagle. The beating of its wings cause the winds in the world of men. At the root of the tree lies a great dragon, Niðhǫggr, gnawing at it continuously, together with other unnamed serpents.
If excess water from watering the World Tree falls on Earth and the wings of an eagle in the tree cause literal winds on Earth, then it's clear that many, if not most ... if not all, people took this to be a literal physical explanation of things. The great dragon gnawing at the root of the tree obviously explains why earthquakes occur.
It just has the Gods up in the sky (where you conveniently can't go and check) like most other beliefs. The "tree" holding heaven up isn't a metaphor for "alternate realities" it is in fact a hamfisted way of explaining why the Gods just don't
fall down. The fact that you can't go and see the tree doesn't mean it was clearly understood as metaphor any more than the Bible belief that there were physical foundations holding up the firmament of the sky.
Viking Priest: "The Gods live up in the sky"
Skeptical Viking: "Uh, wouldn't they just
fall down, then?"
Viking Priest: "Well it so happens, there's this tree ..."
Skeptical Viking: "Why can't I see this tree, then?"
Viking Priest: "Well the base of the tree is far away, past the icy wastes ... you can go check, if you like"
Skeptical Viking: "Uh, no thanks, I'll take your word for it."