Might as well go a bit Norse-Nerd and throw out some predictions on how Oda may end up adapting the "9 Realms" into Elbaf.
Ground Level:
Midgard (Realm of Humans) - The area that we saw Kidd approach; where Shanks and his armada were docked; also likely the area Big Mom's Flashback took place with Mother Caramel's home base. We can't see it in the big spread of the island, but presumably we're looking at 1 half of the ground level suffering a harsh winter landscape, while the other side remains mostly mild with mostly flat landscapes. Rather than being a full "Realm of Humans," it's probably just acting as the main entrance and exit to the island.
Niflheim (Realm of Ice, Mist, and Snow) - With the frozen landscape and "sleeping mists" out in the waters on this side of the island, seems pretty fitting to represent this realm. Usually isn't much to this realm besides being one of the more ancient/primordial ones. Overall it just kinda exists and blends in a lot with the next realm.
Helheim (Realm of the Dead) - Between the castle jail and the act of tying Loki up around the base of the tree, it seems like Oda is also implementing Helheim into this area. There is usually a good bit of overlap between these 2 realms, with some saying Helheim directly taps into Niflheim's cold to make the realm worse/more fitting for the dead.
I'm going to guess that the branches of the tree probably extent out further to one half of the island, and the coverage is probably what makes 1 side suffer such harsher winter climate compared to the other. There appear to be a couple ways up from the ground level, but before we go there.
Underground
Muspelheim (Realm of Fire) and Svartelheim (Realm of Gnomes) - To explain the giant's ability to even make their huge armaments, I'll assume the Giants have been fairly successful at mining their lands in the past. Svartel being the mining cave system, and Muspel being a forging system taking advantage of some underground magma chambers (like Wano had). These seems the easiest for Oda to skip over and only give a glancing mention at some point. However, they could always serve as a base for Loki to end up temporarily hiding at once he's freed.
Now to return to traveling up. The bridge the East Blue cast are on seems moreso for purposes of traveling down to the prison castle, but convenience for our crew it works both ways. You can see what looks like some cables off to the side that are probably the more natural way to go up to the middle level of the tree.
Middle Level
Jotenheim (Realm of Giants) - Yes, technically all of Elbaf is the realm of giants. I'll be going with the guess that this middle level will be considered the more main home for the giants. I'd guess the majority are probably born, raised, train, and live up here in or outside the castle we can see.
Vanaheim (Realm of Vanir/Lesser gods) - Usually these are more nature and fertility related gods, so it's a realm of nature and overgrowth. Everything between the cannons and castle up on this level could lean towards Asgard, but I'm gonna propose that Oda will be misleading us on this. While the majority of giants are probably here, and we'll probably find Hajrudin working on stuff here, the real biggest aspects of Elbaf will be up top, past the rainbow bridge.
Top Level
Alfheim (Realm of Elves) - While there probably aren't actual elves on the island too, a common aspect of the realm is an abundance of bright light. This might just end up being what they call the canopy of the tree. The level that gets to bathe directly in the light of the sun and maybe even connects with the white white sea.
Asgard (Realm of the Gods) - We can see a small bit of the rainbow bridge on the level below. This likely implies that the actual most important land to the giants is up above in the top layer. Whether there's a castle up there too where the main giant royalty live, or it's just a place where they hide away their most important valuables and historic secrets, we'll have to wait and see.
And that's enough nonsense overspeculation off my nerdy mythology care for awhile.