Josh Centers, TidBITS:
An app was either on the Home screen, or it wasn’t on your iPhone. If you couldn’t find an app on a Home screen, a search was your only recourse.
The App Library changes that. All apps live in the App Library, and they can also appear on your Home screen. That means you no longer have to clutter your Home screen with apps that you want to keep installed but use infrequently. You can hide them away and keep only the apps you regularly use on your Home screens. In short, the App Library becomes “the truth” about where you can find any installed app.
This is a pretty major change from iOS 13. Worth reading, and worth noting that you won’t find this on iPadOS 14.