12/12 is not carved in stone. It has been adapted as the defacto standard because it works on the majority of strains grown and it's simple. Take a pure landrace sativa and veg it for awhile, then switch to 12/12 and it will contiune to veg and not flower because where it comes from that's what it normally sees. It will flower when it wants to, and it is not all photoperiod dependent.

In flowering there are 2 important dark period amounts; how long of a dark period does the plant need to induce flowering and then how many hours of dark does it need to stay in flower. And they are usually different.

As to YouTube videos and posts on grow forums, look/read them for what they are and then try varifing that info against known science, either in a book or scientific paper or some reliable source (like university or college resource's online).
OM