Sorry I am very uneducated with Vietnamese. The app teaches you "bạn" means you though right? And what i think im getting out of my learning is bạn may mean one thing but the meaning can be changed just by adding something in front or behind or something? Like one word means one thing but with another word they work together and mean something else?
Yes, "you" is a singular pronoun but we can use "you" when speaking directly to more than one person.
Các bạn = you (speaking to a group)
Các bạn ấy = they
As in the example if you met 3 women at the train station and wanted to address them all.
"You're early!" "You look great!"
I'll take a stab at this. Your sentence is "You(singular) are the women". The literal translation of "Cac ban la phu nu" is "You(plural) is woman", but "is" transforms to "are", and "woman" becomes "women" due to "you" being plural. Whereas if the same thing happened with your sentence, the whole thing would become singular.