I would like to make an iOS app for iPhone as my submission, but I’ve seen people say to make it for iPad or Mac.
Is it required or just suggested that I should make an iPad/Mac app
Not required but it extends your potential user base across the Apple ecosystem. Consider Apple Watch support also but it is not required.
The exact requirement is (slightly edited):
Your app playground must be built with and run on Swift Playgrounds 4.2.1 (or later ) or Xcode 14 on macOS 13
So Playground is mandatory.