An (iOS App Store Package) is the file format used by Apple to distribute apps. Usually, you download these through the App Store, but an IPA Library acts as a third-party repository. It allows you to manually download and install app files that are no longer available on the App Store or modified versions of apps with extra features.
To establish a portable workflow where you can inject IPA files from a computer to your iOS 9.3.5 device, these are the top recommended tools: