Nubia shares the blessings of all the Amazons which is the immortality and the super strength/speed which people keep forgetting they all share. I think Perez established them as being blessed by the goddesses too but with Demeter specifically tossing in the god-like strength and durability?
As for Donna....Really depends on continuity which I can't keep track of. Pre crisis it was the Amazonian strength training and being hit by a version of the purple ray.
Then she got her powers from the Titans.
Then she got her abilities from being a Darkstar
Then she got her powers because shes a reflection of Diana
Then she got her powers because shes every Donna in the multiverse
Then she got her powers from being magically born of clay from the remains of a dead Hippolyta
Im honestly not sure what her deal even is. Though I confess Donna Troy is not a character I really understood or gravitated toward.