Use of AAD from AES-GCM

I'm working on a for-fun cryptography project in Java. I'm struggling trying to understand Additional Associated Data use. From Cipher documentation and further online research I understood that AAD are appended to encrypted data and authenticated, but not encrypted. So, they should be readable without encrytpion key. But I can't understand how, since they seem to be retrieved after cipher initialization, that requires encryption key! Can someone help me? Thank you all in advance!