I can explain how I THINK it explains it in the manga (Spoilers ahead) the descendants(students) are the ones that have great great whatever parents that SURVIVED the final battle and went on to have kids. Ex. Tanjiro, Nezuko, inosuke ex. They look a little different from their great great grand-parents. The reincarnations are the hashira (teachers), because the hashira didn’t survive to have kids. The only ones that survived the final battle are Giyuu, Sanemi, and tengen, and G & S got demon slayer marks, so they would have died soon enough after (two years, I believe). The only one who could have had kids is Tengen. Just thought I might explain a little because everyone seems a little confused.
