What features have been added to the Files app since it was introduced with iOS 11?
iOS 11 introduced the Files app.

iOS 12 added the following features, summarised from


https://support.apple.com/HT209084


AR Quick Look support



iOS 13 added the following features, summarised from


https://support.apple.com/HT210393


  
  External drives supported in Files to access and manage files on a USB drive, SD card, or hard drive
  SMB support to connect to a server at work or a home PC
  Local storage for creating folders on the local drive and adding your favorite files
  Zip and Unzip support for creating and expanding Zip files
  

https://appletoolbox.com/everything-in-the-new-files-app-for-ipados/


Document scanning previously only in Notes, now available to scan to Files directly.
Column view (iPadOS).
Multiple Files app windows (an iPadOS feature supported by Files app).