Special chakra sprouting from the brain of a Hyuuga and going into their eyes? We know how the Sharingan works, Tobirama explained it very well. Please, show me the scan that explains how Hima awakened a Byakugan because of special chakra sprouting from the brain.
No scan. Just basic context clues.
The fact of the matter is that Himawari awakened the byakugan and switched between byakugan and normal eyes in the same way that Uchihas do it with sharingan.
You can call BS or asspull all you want but at the end of the day that just comes down to meta (out of universe) reasons as to why Hima awakened the byakugan.
Calling asspull doesn't explain anything from an actual in universe perspective (the characters themselves don't know Masashi Kishimoto and his staff).
It is most likely the case that all dojutsu result from special chakara in the brain (but different types for different dojutsu).
What other explanation (in universe) would there be? You can't just answer this with "asspull" or "fanfic" either because those are out of universe reasons, not in universe.
Anyway, Tobirama never said that the sharingan was the only dojutsu that resulted from special chakara in the brain. He just said that this is how it happens, so I don't know why you're acting like the sharingan is the only dojutsu that can result from special chakara.
Also, why are you so fixated on the special chakara being unique to sharingan? You're acting like it would weaken or take something away from the sharingan if byakugan was also the result of special chakara. Either way, the byakugan would still be the same relatively weak eye and the sharingan doesn't lose any power.
Are you really just more willing to accept that the byakugan and other dojutsu just come from spontaneous generation rather than having an actual anatomical source such as the brain?