vCenter Server communicates with the ESXi host through a vCenter Server agent, also referred to as vpxa or the vmware-vpxa service, which is started on the ESXi host when it is added to the vCenter Server inventory. See the figure below.
Figure 29. Communication between vCenter and ESXi hosts
Specifically, the vCenter vpxd daemon communicates through the vpxa service to the ESXi host daemon known as the hostd process. The vpxa process acts as an intermediary between the vpxd process that runs on vCenter Server and the hostd process that runs on the ESXi host, relaying the tasks to perform on the host. The hostd process runs directly on the ESXi host and is responsible for managing most of the operations on the ESXi host including creating VMs, migrating VMs, and powering on VMs.