There are several resources on the Internet related to using Google calendars with Thunderbird and its Lightning extension.
However, I did not find anywhere how to use shared calendars as CalDav calendars, in order to avoid using Thunderbird's extension "provider for google calendar". I had to get rid of said extension because I had "unresponsive scripts" in Thunderbird that caused Thunderbird to become unresponsive for several seconds (sometimes up to one minute), which is a PITA, especially when you are writing an email.
In order to use a shared google calendar as a CalDav calendar, one has to know the calendar's URL.
The URL is this one:
https://apidata.googleusercontent.com/caldav/v2/CalID/events
Now, what is this CalID ?
This is the ID of the calendar that one can find on Google's calendar properties page.
In order to get this ID, you must open your Calendar settings. The ID is near to the Calendar address, like in the following screen capture:
In the case of a shared calendar, the ID is likely to be something like:
4alvrbb94gc7sffjki8klf4l2k@group.calendar.google.com
With such an ID, the URL to use would be
https://apidata.googleusercontent.com/caldav/v2/4alvrbb94gc7sffjki8klf4l2k@group.calendar.google.com/events
Now in Thunderbird with Lightning extension, one just just has to create the corresponding CalDav calendar as a network calendar:
And then the calendar and events will be available in Thunderbird's calendars
