I have an iPad developed using UIKit and storyboards now I have to develop UI for iPhone. Designs for iPhone app are completely new from iPad app also navigation is different. I have question regarding should I make different view controllers for iPhone and iPad and different storyboard
I would write the iPhone app in Swift and SwiftUI, and fall back to UIKit if there's something you cannot do in SwiftUI. You may then find it's very easy to extend the SwiftUI stuff to the iPad, and you'd end up with a more modern app altogether. I would avoid using storyboards, they're old and difficult to use and maintain.
But, it's up to you.
It all depends on the app, but in general you should be able to make the app work well on iPhone using the View Controllers and Storyboards you already have.
You want to familiarize yourself with some older less fashionable concepts, namely Adaptivity. If you can find any old documentation on building a "Universal App" for iPhone and iPad, you'll want to read up on that as well. You'll also probably want to understand all the features of UISplitViwController.