Since Broadcom shifted to a 100% channel sales model, you generally cannot buy a license key directly from VMware. You must go through:
This is a full-stack platform. It licenses ESXi, vCenter, vSAN, and NSX as a single subscription. Hybrid cloud and data center modernization.
Download the ISO for your ESXi version (8.0, 7.0, etc.), install the hypervisor, and assign your license key as described above.