That's
@Salah WG 's theory, if I understood it right. The Elbaf tree is the moon people's passageway to the OP world. They are the original kingdom, which exists to this day and keeps its distance as long as the WG doesn't threat the planet's integrity. These giants created different races of smaller people based on their roles, ranging from God's Guardians (Lunarians, the original Seraphim) to the half men half giant Buccaneers, who predated the pirates, but shared their want for freedom.
The little giants sailed the world, conquered places, formed the AK.
Some giants themselves explored around, being the predecessors of the continent pulling onis.
The rest stayed in Elbaf.
Then the AK had its own internal incident leading to the Void Century, and the rest is history--or lack thereof.
It's just a Possibility imo, however what i believe is that there was:
One Ultimate Kingdom (Basically if Garp was a Kingdom)
Followed by Three Legendary Kingdoms (Basically if Luffy - Ace - Sabo were Kingdoms)
Followed by Nine Major Kingdoms (Basically if Nine Straw Hats were Kingdoms)
So One Kingdom ---> Gave Birth to Three ---> These Three gave Birth to Nine
I believe Founders of Ultimate First Kingdom are Ancestors of Vikings, Samurai & Knights (Currently Elbaf - Wano - Mary Geoise)
If Luffy Inherited his Will from Joy Boy Kingdom (One of Legendary Three)
Then Zoro, Nami & Usopp Inherited their Wills from Wano, Mary Geoise & Elbaf
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As for Moon People, i believe They are Humans from Earth, They are Top Scientists & Engineers who were chosen to serve Elbaf, similar to how Wapol made All Doctors work for him (Except Two), and They wore Wings on their Backs to indicate that They are like Elbaf God-Chosen & basically his Messengers/Angels
Then they escaped to the Moon after certain Incident & They built Automata to help them explore Space to journey there & find New Home, but They didn't have enough Energy, so They decided to go back to Earth to search for Goro Goro No Mi
They first established themselves on Skypiea, but 1/3 of them (Skypieans) decided that it's a Safe Place & They don't need to continue on their Mission so They remained there, while Rest continued & descended to Blue Sea, establishing themselves on Jaya/Shandia
But then half of those (Shandians) decided that They don't want to Escape Earth anymore, and They wanted to join Fight to defend it, so They stayed to join Rebellion, while Rest (Birkans) wanted to continue their Mission cuz it was their Dream to explore Space with Automata, and They journeyed around the World (Visiting Vega's Island for example, where They taught them how to build Automata & shared some of their Knowledge, hence why a Scientist from that Island was able to build some Automata exactly like Moon ones)
Eventually Birkans did manage to find Goro Goro No Mi & They were Hiding in Birka (Another Sky Island), but before They can do anything, Enel betrayed them & took the DF for himself & Destroyed entire Island & Defeated their High Priest (Urouge) & then We had Skypiea Arc
This is why when Enel reached The Moon (Which He loved so much cuz He grew up a Birkan), He discovered that his DF is actually the Infinite Resource needed to keep Birka & Automata Permanently On
How do i know that Winged People served Elbaf before going to The Moon?
Simply cuz their City was called "Birka" & They were called "Birkans" which is a Viking Name
And the Fact that They were sad to be forced to go back to Earth, means They were Afraid of not coming back
They clearly Betrayed Elbaf & Escaped to The Moon, indicating that They stole something from them
Whatever They stole should have stayed with Birkans & specifically their Leader/Priest and passed down till it's now in hands of Current Birkan Priest, Urouge!
If you go check Urouge, what do you find? He is Carrying an Unusual Weapon, which turned out to be a Giant Pencil
Why would Urouge be carrying a Giant Pencil with him at all times? It's clearly a Pencil that belongs with Giants
Sky People stole that Pencil from Elbaf & it's the entire reason this whole Moon People Arc exists
All They did was to prevent Giant God from Writing Fate (Just like Pre-One Piece One-Shot)
And it's why Urouge is always Hiding, even Big Mom Pirates don't know where He is