is there any way to transfer files to the files app from a PC via USB?

May I ask what exactly are you trying to transfer?

Without knowing the details, though, I can recommend these 02 options/possible solutions:

1. Use iCloud for Windows (PC). Download iCloud for Windows - Apple Support, then setup iCloud Drive Set up iCloud Drive - Apple Support. Once you have these, you can transfer data from your PC to iCloud Drive. This data can be accessed on iPhone/iPad/iPod/etc. via the Files app Use the Files app on your iPhone, iPad, or iPod touch - Apple Support
2. Since, you mentioned, "there is no way to do that using iTunes": Actually there is, and it is called File Sharing. Certain apps on your iDevices support File Sharing and some do not. Follow these articles to see if they will solve your issue.

• • Transfer files between your PC and devices with iTunes - Apple Support
  • Use iTunes to share files between your computer and your iOS or iPadOS device - Apple Support