Where did you get that information?
"Người phụ nữ" means "woman" and for Duolingo we are putting "the" if they do not put "một" which indicates "one" so is "a" (or "an" for an English word that starts with a vowel.)
I think you are confusing the human classifier "người" with the plural indicator "những". https://www.duolingo.com/skill/vi/Plurals/tips-and-notes
"cô ấy " is "she"
"anh ấy" is "he"
I looked up "ấy" and it said that it is "that" which could be a more specific way of showing that this is a specific woman. https://en.wiktionary.org/wiki/%E1%BA%A5y
However, I know in Korean that without their plural indicator a word can be either singular or plural. Is that the case in Vietnamese as well?