Generally it consists of one central server where clients connect to in a star network topology, also sometimes called a hub-and-spoke setup. The idea being that on the central server you arrange how traffic should flow, and you connect your various devices and networks to it. It does not exclusively have to be used in this way, and it doesn't have to be limited to a single server, but in most simple deployments you'll find a single Access Server where VPN clients connect to, to access resources on the network that the Access Server is on.
Each client device needs a software client program, without it a connection is not possible since the OpenVPN protocol is not built into most operating systems. Such a client program can be obtained from the client web interface of the OpenVPN Access Server. The admin web interface lets administrators configure the Access Server and set up user access, and the client web interface allows user to download and install the required software to start and establish the VPN connection.