Virtualmin/Webmin Architecture (GCE)

Before adding a virtual server with VirtualMin on a host computer managed by VirtualMin, a server is running. What is the virtual relationship with virtual servers that VirtualMin will add to this server that is not included with Virtualmin? You can see the architecture I understand from the link below. Am I right to understand architecture?

So why is it needed to be done in this way? It made me irrational. Looking at virtualmin from the point of view of many virtual servers running on the server, there will be more superior server than all virtual servers. What is the reason for this preference?