Mail app -> Calendar app integration: how does it work?

The description in the app store says,

Integration with other Nextcloud apps! Currently Contacts, Calendar & Files – more to come.

When I get an email with an ICS as an attachment, my only options are to download it or store it in files. Is there a way to integrate mail with calendar so that the calendar gets updated when I receive an invitation?

You can import ICS if they have the correct MIME set by the server. If you can only download then the MIME is most likely wrong.