macOS, or more specifially since iTunes v12.7, no longer stores the IPA files.
There are a number of ways to find an app's IPA, such as the many untrusted, dangerous sites with their signed IPA.
Be safe, read on.
1. Download Apple Configurator 2
This is Apple's app for configuring multiple devices for schools and businesses.
Download it from Mac Store here.
2. Sign In
Go to the app > Account > Sign In using your personal iTunes account.
3. Update The Apps
Go to Action > Update, and select the apps to update (or all).
4. Find in folder
The IPAs will be here:
~/Library/Group Containers/K36BKF7T3D.group.com.apple.configurator/Library/Caches/Assets/TemporaryItems/MobileApps
Note that they are in a cache, which means it will be deleted anytime, so copy it to somewhere while it is there.