From VMware vSphere Essentials Kit and Essentials Plus Kit Datasheet

KEY FEATURES AND COMPONENTS
VMware vSphere Essentials Plus Kit includes
the following business-continuity features and components for always-available IT:
• vSphere High Availability provides costeffective, automated restart within minutes
for all applications in the event of hardware or OS failures.
• vSphere vMotion enables live migration of virtual machines across servers
with no disruption to users or service loss, eliminating the need to schedule
application downtime for server maintenance.
• vShield Endpoint* provides agentless
antivirus and antimalware protection to secure your virtual machines.
• vSphere Replication provides low-cost
replication for your virtual machine and serves as the foundation for leveraging
a cloud-based disaster-recovery solution from a VMware cloud service provider.

Are those features available within the vCenter Server Essential Plus cluster only or can also be used between vCenter Server Essential Plus cluster? i.e. cross vCenter Server Essential Plus cluster vMotion?

Assuming all are licensed as essentials+ then you should have this functionality across clusters.

To vMotion powered-on virtual machines with the Advanced Cross vCenter vMotion feature, you must have a vSphere Enterprise Plus license.
To migrate powered-off virtual machines with the Advanced Cross vCenter vMotion feature, you must have a vSphere Standard license.