iPhone app using iPad|built app for both iPhone n ipad

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I seems to not getting what I query online.my question is,1)if I wanted to built iPhone app ,can I just buy an old ipad.2)without using responsive design,just place button ,image ,etc,any how.can these arrangements built for both iPhone and iPad app?waiting for an answer;thanks in advance,tst

3)for iPhone app building,the initial work,still have to fill forms and sent to apple?now, electronic forms?how long will it takes for this process?

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  1. Yes, you can test on an old iPad - just make sure it is not too old. It should be running iOS 11 or later.
  2. If you want your apps to look good on different screen sizes, you need to use responsive design. The app needs to adapt to the screen size.
  3. If you want to make a native app, you need to register in the Apple Developer Program.
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Thanks for your timely info! ,tst