Adding files to iTunes from files folder

1. iPads don't have iTunes. They have an iTunes Store app and a Music App.
2. You cannot add anything to the iTunes Store, and to add content to the Music App it must be either purchases from iTunes, downloaded from the Apple Music subscription, or synced from the iTunes application on a PC or Mac running Mojave or earlier or Finder on a Mac running Catalina or later.

That is to say, you cannot add music from the Files app to the Music App on an iPad.