How To Create Folder In the App Library

The App Library sorts your installed Apps into multiple automatically curated App-groups - based upon App category information from the App Store.

While the App Library cannot be directly edited, you can create App Groups of your own choice on any Home Page - this being fully described in the iPad User Guide:

Organize your apps in folders on iPad - Apple Support

More information about the iPad App Library are organisation of your Home Screens can be found here:

Use the App Library and Home Screen to organize your iPad apps - Apple Support

The App Library really doesn't have any customization options. However, you can make folders on your Home Screen:

Move apps on your iPhone - Apple Support