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There was a war, or a mass extinction event, and after the war the bodies piled up, or there was a mass grave. At the top of this tomb, a tree was planted, from which the fruits of the devil come.
The tree extracts the Haki (which everyone has although in a small amount) of the same wavelength from the bodies and puts it in a single fruit, that’s why there are not 2 devil fruits naturally identical.
Paramecia fruits are made like this, if there are enough dead animals of the same species – boom a Devil Fruit. Zoan fruits come from dead animals of the same species. Logia fruits come from dead Lunarians, which are fewer in number, which is why Logia are rarer.
Ancient zoans are the result of the root of the tree going deeper and falling on an oil well. That’s why they are more powerful since there were more dead dinosaurs to get Haki out of them.
The mythical zoans are different. They are made of a single dead creature that has very strong Haki. This is why they are the rarest and most powerful.
The general rules of fruit
By eating 2 devil fruits, the person explodes because of the Haki which accumulates in him. Upon dying, the Haki is transferred to another fruit of the same type, since it is bound to that type of fruit by the tree.
They are called the fruits of the devil because they are “the fruits of demonic acts” (war, mass extinction).
A single bite is enough because humans are a favored host for Haki, so all Devil Fruit Haki is transferred through “osmosis”.
Effects can be negated by Haki, if Devil Fruit Haki + User’s Haki is weaker than opponent’s Haki.
Awakening is a process in which it is possible to use a Devil Fruit as Haki, and it is initiated when a user runs out of your own Haki, but not the Devil Fruit Haki, which maintains in life in this case, and which merges with the user’s Haki.
It is possible to create multiple Devil Fruits of the same kind by putting enough Haki of that kind into one fruit, it just happens that the tree puts all the Haki of the same kind into one fruit, and the fruit is ripe after all that Haki has been extracted. Vegapunk failed because the wavelength shifted a bit, and turned Momo pink, but it could have unexpected effects for other Devil Fruits.
This also explains the elemental effects of Haki like Sanji’s fire and Zoro’s Asura.
Basically, Devil Fruits are a cheap way to get powerful Haki.
Haki reigns supreme.
Tl;dr: Devil fruits are the Haki of many dead people in a fruit extracted and concentrated by a tree.