This is a sarcastic expression so for example: Person #1: Hi, I'm a human I have 2 arms and 2 legs Person #2: You don't say :O You don't say / REALLY??? (in a sarcastic way)

It isn't usually sarcastic. It's old-fashioned. It expresses great surprise, and often some shock or horror. The idea is "this is so amazing, I can't believe I heard you correctly, did you really say _that?_" Usually the phrase would be written with an exclamation point. "Guess what I saw." "What?" "Tim and Becca walking together." "You don't say!" "Yes, and there's more. They were holding hands!" "No! You don't say!" "Yes, and I saw him kiss her." "Well! How about that!"

Not sarcastic. It means that what you just said has surprised me. - I spent last weekend in Paris. - You don't say!

YOU DON'T SAY, isn't necessarily sarcastic. It's used in polite conversation with the meaning "Really??" For example, when meeting your new neighbor, you ask him where he's from and he says Russia. Being Russian yourself, you answer "You don't say! I'm Russian too." It's a very common response to a happy coincidence.