What's the B12 content in fermented rice / gruel (eg. shown at https://www.youtube.com/watch?v=ylKMMCxXZI8 )?
Internet search yields conflicting information, even regarding other fermented Indian food like idli and dosa.
The video makes a claim that is incorrect (both in the video and in the description). The wrong claim is that B12 produced by intestinal flora is absorbed by the body. The implication is that consumption of probiotics will aid B12 deficiency by increasing B12-producing intestinal flora. Unfortunately it seems to be the case that while some intestinal flora do produce B12, this B12 produced in the gut is not absorbed and does not prevent B12 deficiency. There is also no evidence that I know of that fermented foods can effectively prevent or treat B12 deficiency.
I strongly advise vegetarians and especially vegans to take B12 supplements. B12 supplements are inexpensive and reliably treat and prevent B12 deficiency. Even if some fermented foods may sometimes contain B12, there is no fermented food that has been shown to reliably prevent B12 deficiency. There may be many health benefits to eating fermented foods and they may be very delicious to eat, but they should not be relied on as sources of vitamin B12.