All ghusls (ritual baths) are Sunnah except for the ghusl of janabah (sexual impurity), and ghusl for janabah suffices in place of wudu (ablution), as both are obligations, and the greater one covers the lesser. If one performs ghusl for reasons other than janabah, they should begin with wudu and then perform ghusl, as ghusl does not replace wudu, since ghusl is Sunnah and wudu is an obligation, and a Sunnah does not replace an obligation.
Commentary #2