Customize user cards
User cards can be customized with CSS.
In addition, custom user fields allow you to add custom fields to the user card, configurable at the /admin/customize/user_fields page with the Show on user card? checkbox.
Hide all user profiles
Site settings:
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hide user profiles from public— Disable user cards, user profiles and user directory for anonymous users.
Other customizations
Other customizations are available through theme components:
Related: Quickly view a bit about each other with User Cards