How does this kind of race-based admission actually work in practice when it comes to “mixed race” individuals? (Scare quotes because I’m not even sure mixed ra

How does this kind of race-based admission actually work in practice when it comes to “mixed race” individuals? (Scare quotes because I’m not even sure mixed race is a coherent concept). How far back in time can you go in search of an advantageous ancestry? This whole area seems rife with absurdities.