Ask HN: I'm a rookie building an app and need advice. Can you help?

Vibe coding is a myth, it will take you only so far and will require manual fixes and refactoring before MVP. Learn the basics of and keep learning, say, Swift. https://developer.apple.com/swift-playground/

> Should I actually use a completely blank iPhone for this instead?

Does not make any difference. Every app runs in its own separate environment. Only iOS device "Language and Region" settings affect all apps on a device.

> should I use a more updated version of the iPhone?

Does not make much difference, unless you need to target some new capabilities introduced with a particular iOS version. Other than that, the rest of the API is the same across multiple recent iOS versions.

> Should I just run a simulator on my actual computer

Sure. It is easier and faster to test on a Simulator using a mouse rather than on a device, tapping. Once you stabilize some code under the Simulator, always re-test it on a real device.

> I don’t want to share my idea with others

This will come as a shock, but ideas themselves are nearly worthless. There has been a previous Facebook, a previous Twitter, a previous Office, etc. that has failed. It is only the quality of execution of an idea, the quality of the Product-Market-Fit and the quality of marketing of a product that makes a product a winner. Start by reading books by Rob Walling, e.g., "Start Small, Stay Small".