Google Calendar: Open Google Calendar → Settings → select your calendar → scroll to "Integrate calendar" → copy the Secret address in iCal format.
URL format: https://calendar.google.com/calendar/ical/YOUR_ID/private/basic.ics
Apple iCloud: On Mac, open Calendar → right-click a calendar → Share Calendar → copy the public URL (ends in .ics).
Public feeds: School calendars, sports schedules, holidays — just paste the .ics URL.