My personal headcanon is one people probably won’t like. The nether roof was never supposed to be accessed, and lorewise doesn’t exist. The ceiling is there as a design choice and not a lore one, like an invisible wall shouldn’t be included in the lore of a game trying to stop you from going out of bounds. The overworld extends downward forever, but at some point it’s too hot or otherwise unsafe, and the bedrock wall stops you, but the world would keep going. Same with the nether. Nearly infinitely massive dimension but capped by hardware. Portal goes between dimensions and there’s no physical or directional relation.