[cromka]

I don't think that's entirely correct? Calendars are part of the OS: https://developer.android.com/identity/providers/calendar-pr...

They get stored in a system table and you add accounts this way as well, right? This is also how DavX provides CalDAV support: it downloads the calendar and syncs it with the system tables.

In any case, at no point was I presented with an option to choose a default Calendar app or nowhere do I see a way to change that.

[lern_too_spel]

You would only get presented with an option when an Intent has multiple possible handlers. If you install a new app that handles the insert event intent, then you'll get prompted the next time that intent is started. I just verified this by opening an ics file after installing a new calendar app.