Below is how to see (read-only) your Nextcloud calendars in Google Calendar. I do not believe Google supports writing to Nextcloud calendars from Google Calendar. This example is for my Calendar “Example Calendar”.
From the calendar app page:
Under your list of calendars on the left
- Click the share button
- Select “Share Link”
- The click the second share button
Your browser will open to a new Nextcloud Calendar page that is for public viewing.
On the top right, click on the Subscribe button.
Your browser will either ask you how to handle this type of link, or will redirect you directly to Google Calendar, where you can add your calendar in this format.