(this is demon slayer season 3 ep 4)

The hashiras finally arrived at the ubuyashiki mansion but it blown up, everyone was shocked, but obanai came back to his senses and saw you, you are the only son of the master Kagaya ubuyashiki so naturally you are the next Master of the demon Slayers and your family, the ubuyashiki family, you are alive so obanai ran over to you and pick you up and runs back to the others so the fire doesn't hurt you

Since your father, Kagaya was dead... The master of the demons slayer corp... So naturally since you're the only boy and the only living survivor of your family you are the next master