First of all the episode was so Cool And Awesome and the people are wondering about Boruto’s “Tenseigan” and how it could be since Boruto needs otsutsuki chakra!
Here is where my theory begins, in the episode where Boruto Uzumaki sees Denki walking all by himself he notices a purple serpent like a snake bite him when his dojutsu activates! Well if anyone noticed, that snake resembles the same snakes that appear when the Mitsuki activates his sage mode! Like the resemblance is uncanny!
And then the Mitsuki looks exactly look like Toneri, again another uncanny resemblance! And as everyone is aware, Mitsuki was created by Orochimaru to be the perfect shinobi! My guess is that Orochimaru got cell’s from Toneri somehow and the based Mitsuki out of him, giving Mitsuki Otsutsuki Clan chakra! Mitsukis sage mode and Toneri’s tenseigan mode look Like similar as well!
Boruto simply has the byakugannbecause of his mother Hinata. Hinata already was powerful and wasnhailed as Byakugan princess, but her kindness got in the way of her fighting capacity. Also, Hinata received chakra from Hamura Ootsutsuki. I’m guessing that’s a factor of Boruto’s tenseigan, since both Boruto’s parents have 6p chakra.