What interfaces are used for ZAPIs?

There are two, depending on whether the API operates on Cluster-owned objects or Vserver-owned objects:

ontap-cluster APIs operate on Cluster-owned objects, like disks, aggregates, and other physical resources. These APIs are executed against the cluster-mgmt LIF using Cluster Admin credentials.

ontap-vserver APIs operate on Vserver-owned objects, like volumes, protocol services, etc. These APIs are either executed against a "vserver management" LIF (A LIF in the Vserver with role=data and data-protocols=none) using Vserver Admin credentials, or against the cluster-mgmt LIF with Cluster Admin credentials and a Vserver tunnel argument supplied on the connection.