If you provide your own custom generated ICS file hosted at a URL, it’s nice to be able to give Google Calendar users an easy way to subscribe to that feed.
The format is
https://www.google.com/calendar/render?cid=webcal://followed by your URL. For example:
https://www.google.com/calendar/render?cid=webcal://pretalx.com/pycon-au-2020/schedule/export/schedule.ics. This drops the user into the Google Calendar interface with a prompt for them to confirm their subscription.
I believe this only works if you are serving your ICS feed over HTTPS.