Recently, Apple updated iTunes for the Mac and PC which rid the application of integration with the App Store. This was a move to make the app lighter and less of a bother to use. While version 12.7 of the software has been well received by many, some users still may prefer using their desktop to install apps on their iPhone or iPad. Luckily, there’s a version of iTunes these users can download to get the functionality back: version 12.6.3.
You’re correct if you looked at that version number and realized it was never released to the public. Rather, it was released to folks such as app developers who may need to access the App Store from their desktop while working on their apps. Fortunately, for everyone, the version isn’t exclusive to certain people and can be downloaded by anyone who wants to bring the old, arguably bloated experience of iTunes back to their computer. Just note that you won’t receive any further updates with this version of iTunes and Apple won’t offer technical help for users.
You can download iTunes 12.6.3 with App Store integration and compatibility with the new iPhone 8, 8 Plus, & X and iOS 11 here.