Learn Italian: Ciao, zia! Come stai? - Hi, auntie! How are you?

When asking about someone's health or how they are doing generally, Italian uses the verb stare (to stay/to be), not essere. That is why "How are you?" translates to Come stai? and never Come sei?.

How do we know it means "How are you?" when there is no word for "you" in the Italian sentence?

The verb ending gives it away! Stai is the tu (informal "you") form of the verb stare. In Italian, you usually drop the subject pronoun (like "I" or "you") because the verb itself already shows exactly who you are talking to.

Why use the informal stai with an aunt instead of the polite/formal sta?

In modern Italian, you almost always use the informal tu (and its verb forms) with family members, regardless of their age or generation. You would reserve the formal sta for strangers, elders you don't know well, or professional settings.

Why is there no word for "my" before zia?

When you are speaking directly to someone to get their attention or greet them, you drop possessive words like "my" (mia). You just say zia, much like how in English you naturally say "Hi, Auntie!" instead of "Hi, my aunt!".