In a proxy server, the servers do not know which client is making the request.
A VPN server is a type of proxy server which allows a client to hide their actual IP address from the servers.
Reverse proxy server
In a reverse proxy server, the clients do not know which server is responding to the request.
A load balancer is a type of reverse proxy server that distributes its traffic to available servers to process requests. The clients wont know which specific server has responded to their request.