u/Alarming-Course1084

Hi I'm kinda new to the game, and I usually play it casually until recently. Can anyone explain how sparks work? Ignoring the mile, why is the 2* speed from ayabe is better than the 3* speed here? And how do legacy work exactly? Is a single 3* + 1* sparks legacy is less better than 2* sparks 2 legacy or even the raw stats distribution from sparks are RNG?