"When will aunt come?"
Translation:बुआ कब आयेंगी?
They are different names for different aunts.
मौसी - Mother's sister.
मामी - Mother's brother's wife.
ताई - Father's elder brother's wife.
चाची - Father's younger brother's wife.
बुआ/फूफी - Father's sister. Both these words are synonyms and there is regional preference for one or the other.
I dont know if this is covered in the notes because i am on mobile and cannot see any of the grammar notes, but i think future versions of the course should cover all these terms at once under the unit for "family," it is a bit confusing for an English speaker if you dont know that hindi makes those distinctions