The Earth in Marvel and and the heroes there were created by The Omega Council with the use of the Concordance engine. The engine literally pulls ideas and miracles from infinite possibilities and universes and merges the best ones into Earth and its heroes
Shortly after Earth was created, the elder god Demiurge was born. He in turn, showered Earth with his power, and made the elder god Gaea.
Gaea (earth) was said to be a primordial concept before concepts were even a thing
Gaea later in turn took the Demiurge’s power and gave birth to Atum in order to protect Earth from all the other elder gods. Atum killed and absorbed all other elder gods except Chthon and Set, who fled the earthly planes in fear of him. Having grown so strong, Atum then went and fused his being to the sun.
This also explains why the sun in Marvel is so powerful, as it is the essence of Atum, an elder god more powerful than Thor and could solo the likes of Set and Chthon (Set himself was a whole metaphysical multiiverse sized snake)
In fact, Atum (Marvel's sun) > his Demogorge form > Utgard-Loki (who considers GoS Loki his shadow)
Marvel's earth can literally resist the First Celestial trying to tear it apart.
