Is the Hadd penalty to be carried out on a young man who confessed to fornication multiple times with different women based on the number of occurrences or only once? And does this count as an expiation for him? And is inserting a finger into the forbidden private parts considered fornication?

A Muslim must stay away from forbidden things. If one is afflicted with any of them, he should conceal it with Allah's covering and not bring his matter before a judge. Inserting a finger into the private parts is not zina (fornication/adultery) that necessitates the prescribed punishment (hadd); for the zina that necessitates the hadd is the insertion of the glans or its equivalent into a forbidden private part. However, this act is a sin, and it is considered among the zina of the limbs mentioned in the hadith (definition): "The son of Adam has his share of zina decreed for him, which he will inevitably attain. The zina of the eyes is looking, the zina of the ears is listening, the zina of the tongue is speaking, the zina of the hand is seizing, the zina of the foot is stepping, and the heart desires and wishes, and the private part confirms or denies that."