Actually there are two different platform where you can run the vCenter Server components (including the PSC): Windows (both physical or virtual) or Linux (only with the vCSA, based on PhotonOS). Initially there was only a Windows version, then the vCenter Server Appliance (vCSA) was first introduced with the release of vSphere 5.0 and has since evolved to become the definitive deployment model for vCenter Server. Starting with vSphere 6.5 the vCSA has become the first choice and has raised the level of vCenter with new functionalities (not available on the Windows version).