VMware vSphere is the virtualization platform as a whole, including the vSphere Hypervisor (ESXi) and vCenter Server. (VMware vSphere Hypervisor is the new/official name for ESXi.) So both terms can't be really compared.
vCenter is deployed on a already existing ESXi host by mounting the ISO and starting the installer - see docs for more information: https://docs.vmware.com/en/VMware-vSphere/7.0/com.vmware.vcenter.install.doc/GUID-F06BA415-66D8-42CD...
Hope this helps!