Yes, most bottom mount refrigerators that are currently manufactured come with an automatic defrost function. But this tends to be the case with only larger refrigerators. Smaller and mini-refrigerators with the freezer on the bottom and the fridge on the top don’t generally have an automatic defrost function.
Refrigerators with the automatic defrost function come with self-heating internal coils. These regularly heat up and melt any frost that may have accumulated inside the refrigerator or the freezer. This is an excellent convenience for you because now you don’t have to spend several hours removing food and beverage items to chip the frost off the inside of the fridge and freezer.
One thing to keep in mind is that the automatic defrost function does use more energy to work. Therefore, your energy bills will be higher with a fridge with an automatic defrost function than it would be with a refrigerator with a manual defrost function.