Learn Portuguese (Portugal): És a rapariga?

In Portuguese, you can usually leave out the subject pronoun (like tu for 'you') because the verb form already tells us who is doing the action. És specifically means 'you are', so we don't need to say Tu és a rapariga? unless we want to add special emphasis.

Why do we use és here instead of é?

És is the form of the verb ser (to be) that matches tu (the informal, everyday 'you'). If you were speaking politely or formally to someone using você, you would use é instead.

How does the sentence structure show this is a question?

It doesn't! Unlike English, Portuguese doesn't flip the word order or add extra words to make a question. The statement 'You are the girl' and the question 'Are you the girl?' are exactly the same in writing: És a rapariga. When speaking, you just raise the pitch of your voice at the end to show you are asking.

Why is it a rapariga and not o rapariga?

In Portuguese, nouns have a gender. Rapariga (girl) is a feminine noun, so it must be paired with the feminine definite article a (the). If you were talking about a boy (rapaz), you would use the masculine article o instead.