Your iPhone app developer will not be designing the app. You need a leader, a project manager to provide accurate details on the following points.
Objective and Outcomes
Design and Functions
Types of Features
Usage and Scalability
Database Integration (what database your app will connect with)
Security
Access Options
Based on the information, a team composition is decided. The work of iPhone app developers is restricted to all programming activities (front-end, back-end, functionality, integrations, APIs, etc.)
If you find iPhone developers with UX and UI design skills, check their skill sets and their portfolio of design work. You will need specialist designers if you are developing game apps, entertainment apps, e-commerce apps, educational apps for children, or any other app that requires fantastic visuals.