Is the iPhone privacy is secure or not?

Secure against who? Secure against what? How much are you worth to attack?

Nothing is secure against everything. Or can be. Not while remaining useful for its intended purpose.

iPhone does quite well against the sorts of security threats that most of us encounter.

But Face ID can fail against twins and close relatives, for instance. Different folks have different theats.

If you’re an investigative journalist, a political dissident, a high-profile or controversial person, a senior government official, somebody with access to classified or military secrets, maybe an iPhone or iPad isn’t for you. Or maybe an iPhone or iPad will work, but with some usage restrictions. For many of the rest of us, iPhone does just fine.

There’s no absolute security. Never has been. Never will be.

Here’s some reading: Apple Platform Security - Apple Support (also available as a PDF.)

Yes. If you have a passcode enabled or Touch ID / Face ID, its very secure.

Why do you ask? What are you afraid of?

Apple Platform Security - Apple Support

Use the built-in security and privacy protections of iPhone - Apple Support

Not sure what area of privacy you are talking about, but no device is 100% secure when it comes to your privacy, not just the iPhones. However, Apple goes through great lengths to provide security, but you are also a part of that equation.