Hello! I am TABRIS, the alter-ego of Rudolph. I am a huge Evangelion nerd and want my own place to talk about it!!
First things first, Evangelion lore means nothing. In Neon Genesis Evangelion, the 26 episode anime + The End of Evangelion, the lore and worldbuilding history is not as important as the connections between the characters. This is extremely important. Evangelion is about emotions and people, not the angels or where they came from. They exist mostly as a vessel of bringing the message of Evangelion: Take care of yourself.
That being said, I love to over-analyze Evangelion lore. It is extremely interesting to me and I love trying to explain it to other people. Does it make any sense? NO! However, I have established that that is not the point of Evangelion.
Billions of years ago, there existed a species only known as the "First Ancestral Race (FAR)." The First Ancestral Race knew of their own doom through means we cannot understand. They predicted their demise, and decided to create a failsafe. Upon their death, their souls would reside beyond the Doors of Guf. This place is where souls exist when they are not existing in the land of the living. Souls leave when a vessel is born to house them, and they return upon the death of that vessel. Knowing their souls would become trapped behind the Doors of Guf if they died without vessels to return to, the FAR decided to create vessels and spread them across the universe. They chose to create two types of vessels; ones deriving from the white seeds and the ones deriving from the black seeds. White seeds housed a creature called Adam, while the black seeds housed a creature called Lilith. Whether these were their given names or what the humans of Earth dubbed them is unknown. White seeds housed power and life, the vessels deriving from Adam being gifted infinite energy from their Super Solenoid (S2) Engines. Black seeds housed knowledge and wisdom, and as such the vessels derived from Lilith are able to die. However, it is this death that allows them to grow and learn. They held power comparable to that of Adam's vessels.
Each seed came with a progenitor giant and a Lance of Longinus. The spear, embedded in the body of the giant, prevented the respective Adam or Lilith from creating vessel. The FAR did not want their created vessels to die within the vaccuum of space on their way to their new home planet, of course. Once arriving at their new home planet, however, the lance would remove itself from the giant, allowing them to create children/vessels. Children of these progenitors are called Angels. Adam's children are each an Angel, being self-sufficient vessels that cannot run out of energy. Lilith's children, however, can die. In order to remedy this, they are able to reproduce with one another. Adam's children are immortal alone, Lilith's children are immortal together. These types of vessels are directly conflicting, not meant to exist on the same planet at the same time. The Lances existed for another purpose: to subdue and deactivate a giant if two seeds were to land on the same planet.
Earth. Billions of years ago, a white seed came to Earth. The respective Adam began to create children, as he was programmed to do. His Lance safely removing itself from his body and existing with him silently. Adam created fifteen children. No more, no less. They would never reproduce or die, if all went to plan, and as such there was no use for more of them. Each with given a name and an aspect, and they lived peacefully after each taking a soul from beyond the Doors of Guf. However, this peacefulness would not last. Barrelling towards Earth was a black moon bearing Lilith. Under normal circumstances, the Lance of Longinus residing within Lilith's seed would simply not deactivate, not allowing the Lilith to create her children. However, in this case, the Lance that kept Lilith asleep was damaged when the black moon crashed into Earth. This cataclysmic event is known as First Impact (FI) and solidified Lilith's place on Earth. With Lilith's lance damaged, Adam's lance had to activate to disable the giant of life and light. Adam's seed originally landed in Antarctica, and Lilith's seed landed near what would become Japan. With Adam disabled and in suspended animation, the angels were disabled as well. Their souls would not be allowed to return to the Doors of Guf, as their vessels were not dead, only suspended in an eternal sleep. The bodies of the angels fell to rest, and the reign of life as we know it would just be beginning.