I was rewatching the movie, and they were vague on how the Tenseigan occurs.
It's said that you need an Otsutsuki body and Byakugan to acquire Tenseigan. HOWEVER, the entire Otsutsuki Clan, from Hamura, Kaguya, Momoshiki, Kinshiki, and moon Otsutsuki had all had Byakugan, but Tenseigan never...