Can compares her dad’s lightning bolt to lightning lads if she is from the arrow flash earth and legion is on supergirls earth this doesn’t make sense
The different earths are tweaks of the same Earth where history took a different path. Consider the council of Wells. On every earth there is a Wells.
Flash’s Earth will also most likely have a legion. On Flash’s earth however Lightning Lad probably exists in a closer future than on Supergirl’s earth (or maybe time travelled a bit).