You can't import .ics files into the iOS Calendar app

• You can't save it to iCloud files and open it
• You can't use the share button
• You can't open the Calendar app and import it from there

You can only import .ics files if you receive them from Apple's native iOS Mail app!

But you can also open it via Safari.
This tool loads an .ics file and opens it in-browser. All client-side.