Actually, you add the nextcloud account either in Kaddressbook or in Korganizer/Kalender, or in Akonadi konsole. There was something called “online accounts” in Plasma but I never understood what it was good for, mostly obsolete instant messaging services, but not the integration of Nextcloud. KDE/Plasma has nowhere near the consistency that GNOME has. Sure, it generally offers more functions and options, but it is certainly more messy.