Relevant market is not a vague rhetorical definition as you such described. US FTC has not attempted to define a relevant market for digital apps, but EU clearly considers iOS alone as a relevant market and thus regulates it with DMA.

To give you an idea of hypothetical regulation scenario: You want to move to an Android app. Sure you gotta need to change your phone as well. This is likely much more expensive then the app itself, thus it may make Android/iOS apps non-interchangeable. If FTC finds that this is expensive enough so most people won't make such move even if Apple decides to considerably increase the app store tax, then the App Store itself is a relevant market thus monopoly.

We don't know the legal conclusion since FTC hasn't done such study (this is a very complex process that takes several years with huge uncertainties, this is why politicians prefer direct regulation via legislation), but many experts believe that Apple is not in a very safe position.