There's more to this situation than just what has been discussed so far. Let me explain.
In iTunes versions prior to 12.7, whenever a new version of an app was downloaded, the old version was moved to Trash. I could move the old version from the Trash and save it.
This enabled me to revert to a previous version of an app by simply deleting the new version from my iPhone, deleting it from iTunes, dropping the prior version into iTunes and then syncing the iPhone. In other words, it was possible to keep multiple prior versions of an app and install any one of them on my iPhone whenever I wanted.
There is no way to save a previous version of an app with iTunes 12.7. Downloading an app using the iPhone overwrites the older version without the option to save the previous version.
This may be Apple's intent... make app updates the same as IOS updates. Impossible to go back to a prior version.