There are many resources and tutorials mentioning iPad and/or iOS being compatible with Apple's Catalyst framework,
So, the same source-code runs on Mac OS X as well.
But does Catalyst support iPhone?
(Please tell me your personal experience to clarify this, and not that the name of iPhone operating-system is iOS)
Mac Catalyst allows iOS apps to be built for and run on macOS.
So, by definition, Catalyst apps support iOS, and usually the iPhone. The only case in which they wouldn't is if the app was specifically an iPad-only app that runs on Catalyst on the Mac, but was not enabled to run on the iPhone.
More about Catalyst: https://developer.apple.com/mac-catalyst/
You asked for personal experience: I have apps in the App Store for iOS, iPad, and Mac, via both Catalyst and non-Catalyst builds.