Shared info on 2 iPhones that are linked

first off, let me state that I am not tech-savy in the least, but here is my question; I know that there is a way to sync to iPhones, but when that happens, what information can the other person see? I am absolutely positive that my husband has somehow linked our phones, because he asks me questions about content from a text message I shared with people like my daughter or my son or my friends. He asks me questions that make me realize that the only way he could’ve said that was if he had read the messages. Also with emails-he will refer to something in a backhanded way from a discussion within an email that I shared with people so my question is if he has synced our phones then what exact information would he and could he be privy to? I don’t know if it’s the phones being synced somehow or if he has just found a way to snoop thru my phone but either way I’d like to know exactly what he has access to, thank you all for any help you can give me on this