I think a lot of people aren't understanding how calendars work on Android. Android has a calendar subsystem which basically just stores calendar data.
Then you have a calendar APPLICATION with allows you to interact with the data stored in the calendar subsystem.
And finally, you have something to optionally synchronize the data in that calendar subsystem with an external calendar server (CALDAV). The package of choice for this is DAVx5 which synchronizes with any standard CALDAV server, which includes Nextcloud, or even Gooble (does not require any closed source package to interface).
Using the application does NOT depend on the presence of the synchronization system, but it does depend on the calendar subsystem.
Without the synchronization system, you simply will be using local calendars.
And also note that if you install multiple calendar applications, they will SHARE their calendars.