When Apple first released the iPad, apps were still sold separately from the iPhone. This means that if you purchased an app that supports both the iPhone and iPad, you would need to get them both separately. Apple later streamlined this by ensuring that as long as the app supported either device, you’d only need to purchase it once.
This unfortunately did not apply to macOS apps, where apps on the Mac App Store still remained separate from iOS apps. However, according to a recent tweet by developer Steve Troughton-Smith, it seems that Apple is currently working on making macOS compatible with universal app purchases.
This would be similar to how iPhone and iPad app purchases work, where as long as the app is supported on the iPhone, iPad, and Mac computers, you’ll only need to purchase the app once and you’ll be able to download it across all your devices. Of course, this is assuming that the developer has set it that way.
https://twitter.com/stroughtonsmith/status/1225125126427025409
As it stands, there are some developers and apps that are sold separate from each other, meaning that if you wanted the iPhone/iPad app and the Mac version of the app, you’ll need to purchase it separately. There is no word on when these changes will be rolled out, but it might be something to look forward to if you’ve bought into Apple’s entire ecosystem.