It's most both, though some people might use leaves, others the berries. I'd assume at least the grapes are allowed to ferment in order to create the yeast. This method is used in Europe and here for artisanal breads. Most of what you found in your research appears to be all from the same source.

A few plant leaves contain something that helps things ferment. I saw it after watching a bunch of YT videos about fermenting. After all, long time ago, there were no yeast packets for sale.