In the classic canon Alan Scott’s ring and lantern were forged from the Starheart, a gathering of all the random magic in the universe that the Guardians got together to help create order. Now, this universe doesn’t seem to have Guardians or one would imagine we’d have seen an Oan powered Green Lantern by now.
Instead the source of his powers is a bit more…well, obviously named. He’s Earth’s one true knight, and his power source is The Green itself.
Earth 1 has Swamp Thing, Earth 2 has Green Lantern. I can dig that.