Welcome to the Community,
vCenter Essentials is included in the Essentials Kits, and it's limited to a maximum of 3 hosts, and the features which come with the Essentials/Essentials Plus.
The difference between vCenter Foundation and vCenter Standard is that the Foundation Edition is limited to a maximum of 4 hosts (3 host in versions prior to vSphere 6.5 Update 1), and doesn't support Operations Manager, and Linked Mode.
André