https://thenextweb.com/apple/2020/02/06/apple-now-allows-you-to-make-universal-purchases-for-mac-and-ios-apps/

It’s quite annoying when you have to pay two times to use the same app on Mac and iOS. Thankfully, you won’t have to do that anymore. Apple has added the ability for developers to enable universal purchases with the new XCode 11.4 beta dev tool.

As developer Steve Troughton-Smith pointed out on Twitter, if this option is enabled by the app developer, you can purchase an app on iOS and use it on Mac, and vice versa. He also noted that this feature inclusion was unexpected, and was rumored for a release in a couple of years.