With औरत you don't pronounce the 'a' in त, but with औरतें you have an /e/ instead of the 'a' so you have to remove the previous 'a' in र.
I'm slightly confused about the pronunciation of the last letter, can someone clarify, please?
/aurateṅ/ Can you be more specific about the question you have? It's /e/ with nasalization added.
They're synonyms. They both mean the same thing. Duolingo is just teaching you "aurten" first