There was a war, or a mass extinction event, and after the war, the bodies piled up, or there was a mass grave. On top of that grave, a tree was planted, which is where devil fruits come from.
The tree extracts Haki (which everybody has although a small amount of it) of the same wavelength from the bodies and puts it all into a single fruit, which is why there are no 2 same devil fruits naturally.
Paramecia fruits are made like this, if there are enough dead weirdos of the same kind – boom a devil fruit. Zoan fruits are from dead animals of the same kind. Logia fruits are from dead Lunarians, which there are fewer of which is also why Logias are more rare.
Ancient Zoans are a result of the root of the tree going deeper and stumbling upon a well of oil. This is why these are more powerful since there were more dead dinosaurs to draw Haki form.
Mythical Zoans are different. They are made from a single dead creature that has a very strong Haki. That is why they are most rare and most powerful.
If you eat 2 devil fruits you explode from the Haki clash inside of you. If you die the Haki gets transferred to a different fruit of the same kind, since it is linked to that kind of fruit by the tree.
They are called devil fruits because they are “fruits of devilish deeds” – war, mass extinction event.
One bite is enough because human is a preferable host for Haki, so all the devil fruits Haki gets transferred by “osmosis”.
Effects can be negated by Haki, if the devil fruits Haki + users Haki is weaker than the Haki of an opponent.
Awakening is a process where you can start use your devil fruit as Haki, and it is initiated by you running out of your own Haki, but not devil fruit Haki, which keeps you alive in such a case, and merges with your own Haki.
It is possible to create multiple devil fruits of the same kind by putting enough Haki of that kind into a fruit, it just happens that the tree puts all the Haki of the same kind into a single fruit, and the fruit is ripe after all of that Haki has been extracted. Vegapunk failed because the wavelength shifted a bit, and made Momo pink, but this 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 of getting strong Haki.
Haki reigns supreme.
Tl;dr: Devil fruits are Haki of a lot of dead people in a fruit extracted and concentrated by a tree.
Theory by Impressive_Formal444 (https://www.reddit.com/user/Impressive_Formal444)
