You’re right, of course - but at the same time missing the point somewhat; I have a Google mail account that I access from Nextcloud, and as is normal, that same account holds calendar data and contacts data.
I may have expressed myself inaccurately in trying to be more exact what I want to do, but the original question was pretty straightforward; how do I connect (sync is the wrong word, sorry) to CardDAV servers from Nextcloud?
I guess from your answer that this isn’t implemented, and that’s fine of course - volunteer programmers are a finite resource, and we should all be grateful for the effort they put in - but it wasn’t obvious to me that it wasn’t implemented, as mail and contacts are pretty tightly integrated functionality.
I would think this is more of a question of prioritizing where to invest effort? Ideally, if you want to provide independence from the big cloud service providers then good integration is a facilitator; otherwise the transition may be too abrupt, and keep people from taking the leap.
Thank you for your time and answer, though!