I don't really understand the question.

The provider for Google addon has been around for about as long as Thunderbird has had a calendar addon. It is maintained and updated by the author and works much as it has for the last decade with the latest versions of Thunderbird.

In the interim years the use of caldav to connect to google and get calendar data has improved to the point the provider only supplies a very small difference in supported facets (task mostly). With the 91 release Thunderbird had something of a makeover in the setting up of calendars and now automatically detects all calendars at google natively.

So can we back up to the point you actually say what the real issue you are facing is?

Do you have the latest version of the provider installed and active? https://addons.thunderbird.net/en-US/thunderbird/addon/provider-for-google-calendar/

Thank you for your answer. I didn't know that "the provider" was updated to the latest Thunderbird versions. I was under the impression that Thunderbird wanted to do it all by itself now. But installing the provider for google calendar solved the problem. Again it is very simple to make the calendar of my wife visible. It's a matter of marking a checkbox. Without the provider for google calendar I wouldn't know a way to add that calendar. Thunderbird doesn't offer the choice to do that.

If the calendar is in your google account when you log in there. The process is fairly simple in V91 and later. Using the application menu, with the three bars on the toolbar. Select New > Calendar> on the network, enter your email address (google ID) and follow the prompts until you are shown the list of calendars in the email account.

I stopped the "provider" and did exactly what you suggested. My wife's calendar was not in the list to choose. With the "provider" on it just is in the list.

So this is a calendar of another person who gave me permission to see and even edit her calendar. Thunderbird 97 won't show it in the list to choose, the provider for google calendar will.

If the provider for google calendar keeps being maintained this is not such a big issue, but imho it would be better when Thunderbird itself could render such calendars. I hope it will in the future. I see why Thunderbird takes the effort to incorporate this functionality, so many people use (google) online calendars. But I hope the will make the feature-set complete.