An file is an archive file that stores an iOS app. Think of it like a .exe file for Windows or an .apk file for Android. Every time you download an app from the App Store, your iPhone temporarily processes an IPA file.
Finding the file is easy; installing it is the hard part. Apple’s ecosystem is designed to prevent users from installing software from outside the App Store (sideloading).
There are several archives where old IPAs are hosted. Use these sources at your own risk.