A true “portable” library requires strict folder structure:
You can sometimes download compatible versions of apps directly from the App Store without any extra tools if you have previously "purchased" them on your Apple ID. Go to the App Store and tap the Purchased tab. Find an app you previously owned and tap the Cloud icon .
. While modern app support has vanished, these libraries allow users to resurrect obsolete devices by sideloading legacy versions of apps and games that Apple no longer hosts on the App Store. Review: The "Retro Resurrector" Experience