Simple Calendar Pro is a full service calendar app that respects your privacy. By default, this app operates offline enabling you to create events manually or import them from ICS files & receive reminders/notifications.
Most Calendar services like Google Calendar, Outlook, and Nextcloud offer CalDAV syncing to push/pull events to & from your local calendar. A third-party sync app, DAVx5, is needed to handle the syncronization with whatever service you are using.