Opening an .ics file on iOS from the Files app will show the events, but will not allow you to import/add them to your calendar.
Not true. I just tried it on an iPhone 14 with iOS 18.4, and it works fine.
You tap on the ics icon, and it opens up the details. Tap "Done" and the event lands in the calendar you specified.
What can I say, other than we have very different experiences. Opening an .ics file from the Files app indeed shows its details, but selecting "Done" does nothing but dismiss its contents and adds nothing to the calendar.
Contrast this with opening the same .ics file from an email attachment or viewing it online, which presents an "Add All" button.