ZT is an SD-WAN, a specific type of highly complex VPN. Basically it is a hub and spoke VPN, but with advanced detection to allow full mesh ad hoc between nodes when appropriate. But at the end of the day, each link is a standard VPN connection. It's just how it is hosted (central controller) that is outside of the network of connected gear; and the hub and spoke vs full mesh auto detection that makes it so unique.
A nearly identical system is Cradlepoint, but it does not allow the mesh option, only the appearance of it, so that centralized traffic control on the controller is available.