The cosmology of Norse mythology has "nine homeworlds" or "nine realms", unified by the world tree Yggdrasil. The Norse creation myth tells how everything came into existence in the gap between fire and ice, and how the gods shaped the homeworld of humans.
In the middle of Asgard, where the Gods and Goddesses live, is Yggdrasil. Yggdrasil is the tree of life, and it is an eternal green Ash tree; the branches stretch out over all of the nine worlds in Norse mythology, and extend up and above the heavens. Yggdrasil is carried by three enormous roots, the first root from Yggdrasil is in Asgard, the home of the Gods is just next to the well-named Urd, this is where the Gods and Goddesses have their daily meetings.
The second root from Yggdrasil goes all the way down to Jotunheim, the land of the giants, next to this root is Mimir’s well. The third root from Yggdrasil goes down to Niflheim, close to the well Hvergelmir. It is here the dragon Nidhug is chewing on one of Yggdrasil’s roots. Nidhug is also known to suck the blood out of the dead bodies, that arrive at Hel. Four stags—Dáinn, Dvalinn, Duneyrr and Duraþrór — also live in the branches. At the very top of Yggdrasil lives an eagle, the eagle and the dragon Nidhug are bitter enemies, they truly despise each other. There is a squirrel named Ratatosk, and it spends almost the entire day, by running up and down the ash tree.
Ratatosk does whatever he can, to keep the hatred between the eagle and the dragon alive. Every time Nidhug says a curse or an insult about the eagle, Ratatosk will run up to the top of the tree, and tell the eagle what Nidhug had just said. The eagle is equally rude in his comments about Nidhug. Ratatosk just loves to gossip which is the reason why the eagle and the dragon remain constant foes.
According to the Norse religion Asatru, the realms are distributed in the following order:
The three worlds above the earth, in heaven:
1. Álfheimr – home of the Light Elves.
2. Asgard – home of the gods. The Bifröst (The Rainbow Bridge), which originates in Asgard, allows the Asgardians to travel to Midgard and is possible that it allows them to travel to the other seven realms as well.
3. Muspelheim – home of the fire giants and demons.
The three worlds on earth:
4. Jötunheimr (Jotunheim) – home of the frost giants (Jötunns).
5. Midgard – home of the humans also known as mortals. Midgard is located in the center of the nine worlds. It is connected to Asgard through the Bifröst.
6. Vanaheimr (Vanaheim) – home of the Vanir – a group of gods associated with fertility, wisdom, nature, magic, and the ability to see the future.
The three worlds below the earth, in underworld:
7. Svartálfaheim (Svartalfheim) – home of the Dark Elves.
8. Niðavellir – home of the dwarves.
9. Niflheim (Niflheimr) – home of the dishonored dead.
Sources:
https://en.wikipedia.org/wiki/Norse_mythology
https://norse-mythology.org