Seems more like a Zabbix question than a Nextcloud question, and I’m no expert on Zabbix. However I can maybe provide few thoughts that might help to point you in the right direction:
If you just add your Nextcloud server as generic host, I wouldn’t expect Zabbix to be able to monitor much, except maybe ping or HTTP status and the information provided from the status.php page, for which you still would have to manually create the respective queries, I guess.
If you want to gather more in-depth information, you would have to let Zabbix access your Nextcloud via API. Maybe this thread is of any help with this: Monitoring with Zabbix. This might be helpful too: GitHub - nextcloud/serverinfo: 📊 A monitoring app which creates a server info dashboard for admins
There is also Nextcloud template for Zabbix: Nextcloud monitoring and integration with Zabbix
However, the Zabbix forum would probably be a better place for asking questions about how to use and configure their templates.