A vSAN Witness Appliance, or a physical host, can be used for the Witness function. The vSAN Witness Appliance includes licensing, while a physical host must be licensed accordingly.
The Witness OVA can be deployed on an ESXi 6.5 or higher, but the ESXi host CPU must meet the requirements of vSphere you are installing.
Deploying the witness for a normal configuration requires 10GB of cache device. The physical host does not require flash nor SSD device. The capacity device must be 350GB or greater. Traditional spinning drives are enough because the witness will mark those devices as needed.
Each vSAN stretched-cluster configuration requires a Witness host, a witness cannot be shared among the clusters. The Witness must reside on a third site that has independent paths to each data site. While the Witness host must be part of the same vCenter as the hosts in the data sites, it must not be on the same cluster as the data site hosts. Network Address Translation (NAT) is not supported.
NOTE: The Witness host OVA file includes a license, it does not consume a vSphere license. However, a physical host requires a vSphere license.