When I was growing up we always had yoghurt in the fridge. It’s so cheap and simple to make. Some people stuff around with getting their milk to 84.67341°C and then trying to maintain temps while it forms. My Mum would mix a tablespoon of yoghurt into a bowl of milk, cover it in cling wrap and shove it on a shelf out of the way. 8 hours later it was yoghurt and ready for the fridge. No thermometers no special tools.