In the olden days, we could easily pay for an app for a friend to own.
One route for this was through the iTunes app on a Mac. Currently in macOS Mojave, the iTunes app seems to no longer know about apps at all.
Old Answers here on Ask Different talk about using the Share sheet on the iOS device, inside the App Store app. But on iOS 12.3.1, I can find no such item in the Share sheet.
➥ Does Apple no longer allows gifting apps?
I see posts about the Mac App Store not offering the "gift an app" feature. Perhaps Apple has removed that from the iOS App Store as well?