In Guardians of the Galaxy, Vol. 2,
Ego seems to describe his origen as simply realizing that he was alive, then discovering that he could attain immortality through his relationship with a planet that it sounds like he discovered. So there was a period in which Ego existed before he even discovered the planet. There was no mention of his parents, or even of other-universe relations like a brother Alter Ego. It just seems as if Ego had always been alone except for using beings in his project. There is even the idea that Ego himself decided that he was a Celestial based on how people he met described Celestials. Even though his own power seems to be a result of a planet that he discovered.
So, in the MCU, how was Ego created?