Apple released the first beta of iOS 13.4 today and with it an important change to the Apple ecosystem. Starting with that version of iOS and probably the equivalent version of macOS, tvOS and watchOS, developers will be able to offer universal app purchases.
This means that some apps may be available for one price to get the versions across some or all Apple platforms. So you can buy the iOS version of an app and the macOS version for one price at one time. It is important to note that it will be up to developers to enable this, so it may take days or months for apps to appear as cross-platform bundles, and some developers may decide to keep them separate.