You need to stop the formation of ice blocks by stirring the mixture while it's freezing. The more stirring, the slower the freezing, the tinier the crystals, the smoother the ice cream. Plus, the mixing lets tiny air pockets get in the ice cream which makes it much easier to scoop. Without them, scooping would be very difficult. Usually, the higher the butterfat, the richer and smoother the ice cream.
Frozen yogurt uses yogurt which is fermented milk.
Yogurts with higher fat levels make creamier frozen yogurts. Usually this is sugar. Sugar also helps to keep the ice cream from becoming a solid block of ice. Because water that has sugar dissolved in it has a freezing point below 32 degrees, not all the water present in the ice cream freezes. Stabilizers - These are added to protect the ice cream.