To subscribe via Google Calendar:
1. Tap the blue plus + icon in the bottom right of the calendar and follow the oncreen instructions.
To subscribe on an Apple device:
1. Hold your thumb down here and select "Copy Link"
2. Open the Calendar app, select Calendars > Add Calendar in the bottom left.
3. Select Add Calendar Subscription and then paste the link you copied in step 1 into the Subscription URL box.