things like ginger starters and sourdough starters just use natural free floating yeasts. Honest
far as yoghurt goes I believe the anerobic bacteria that creates the lactic acid which in turn makes the milk proteins coagulate, is naturally present in milk anyway. They probably have a digestive purpose in one of the cow's stomachs and get into the milk by some sort of internal process.
I suppose there could be spores floating around, but by their very nature anerobic bacteria are killed by exposure to oxygen. So no free floating ones.
You'll probably struggle to make yoghurt from uht treated milk without adding a culture. Non heat treated milk will curdle all by itself, quite quickly.
Good experiment to try
But simply adding a small amount of natural yoghurt to uht milk will give it all the bacteria it needs to turn.
It is a fascinating idea that we can preserve foods and make them tastier by helping them to rot.