The sons of the priests made the ointment - Only the priests were permitted to make this ointment; all others were forbidden to do it on pain of death; see Exodus 30:34-38 (note), and the notes there.

And some of the sons of the priests made the ointment of the spices. For though the Levites had the care of the spices, they might not make the ointment with them, only the priests, see Exodus 30:23.

And of the priests' sons were preparers of the ointments for the spices. It is the preparation from various spices of the holy anointing oil, Exodus 30:23-25, which is meant, and which consequently was part of the priest's duty.

The ointment - This is added to shew, that though the Levites were intrusted with the keeping of this ointment, yet none but the priests could make it.