I use uht milk plus some active yoghurt to make mine. Then I just keep some of it to start next batch. I bought an easiyo yoghurt maker. Don’t like the prepared satchels. Did bit goggle searching found a recipe I have used for few yrs now. Maybe not as much friendly bacteria as just using fresh milk. Don’t honestly know. Science would probably tell. It works for me. I get a litre of yoghurt out of a batch.
Have nosy at this site plenty of info on different yoghurt methods including one I use.
http://sustainablesuburbia.net/how-to-make-yoghurt-from-scratch-in-an-easiyo-yogurt-maker/