i use nextcloud 18.0.1 with with contacts 3.1.8. There I created an address book for myself and my son. My son’s address book is shared with me in nextcloud. When I create a new contact in the iPhone, it ends up in my son’s (shared) address book.
How can I change that new contacts entered in the iPhone end up in my address book?
That is a functional limit of iOS.
AFAIK you cannot move contacts between addressbooks.
There are apps in Apple App Store which can do that. You have to buy something to achieve your goals.
Have the same problem and move contacts in Nextcloud.