Some babies can begin rolling over on their own to their tummy as early as 4 months of age but some babies don't roll over on their tummy on their own until 6 months of age.
Since babies can eventually roll over onto their tummies when under a year old they should be on a firm hard surface to sleep such as a firm baby mattress.
Never put anything soft into the babies bed with them until they're at least 1 year old or older because they could roll over into the soft objects and not be able to breathe which can lead to death.
If the baby is under a year old and rolls over onto their tummy during sleep it's okay and you don't need to roll the baby back over.
Eventually the baby will be able to roll back over to their backs on their own but it can take some time for that.