content-users relations

Actually, the "User - Profile" Joomla plugin is published too (see screenshot).
This website has Community Builder installed. All CB plugins are published.
Is there another plugin that should be checked / installed ?

This issue was due to the field option "Displayed user" being set (by default) to "Author of the item".
The only way I can get the profile field to work in the backend is when "Displayed user" is set to "Allow selection (in item form)".
Then I can create a Profile item in which I select a specific user and I set the car(s)-user relations.
In the frontend this item shows up the profile field and the "profile-cars-rel" relation field.
In the backend, in the Profile item manager listing, I can add a column "Cars" using the "profile-cars-rel" field. I can Probably add it as a filter too (though for now I can not select it form "Custom filters").
That's nice, but with this method I must manually create a profile item for each Joomla user. That would make the daily maintenance much too complicated and the Profile type related features would not be available real-time to the new user accounts.
Is there a way to get the new Profile items automatically generated when new user accounts are created & activated ?

yes you need to create a plugin to create item after user creation

I see.
Is there a pre-existing plugin, or tutorials that list all the data tables where new entries must be added so as to force-add a new item in FlexiContent ?
Or maybe there's a neat way to call FlexiContent methods that do it in the right way ?

No build in solution
You need to create own
Check tutorial you have api for item creation

hello
the article is here
www.flexicontent.org/documentation/tutor...-in-custom-code.html
in fact you need to create system plugin to listen user creation
docs.joomla.org/Plugin/Events/User