How can I keep my notes private and hidden on iPhone?

Use a device passcode only you know, and an Apple Account password only you know, and don’t share it.

The damage that sharing an iPhone, or loaning an iPhone, can involve is not to be underestimated, too.

No, but you have the option to lock the Notes app entirely, ensuring only you can access with Face ID authentication.

Hold on the app until the pop-up menu appears, then tap Require Face ID.

Hold down the app, afterwards press “require Touch ID”, as long as only you only have the password, no one else can access it.