opc server internals

can you help me with some basics which i thought understood:
1- when server is connected to the plc but no client used is there any data being fetched from plc at all? or there must be at leat 1 client with at at leat one group and 1 item? so when client disconnects plc gets no traffic?
2- is there a concept of server group at all or only client groups exist?
3 - between server and device eg plc what methods of exchange exist - is polling the only option or are there other ways at this level like change of state?
4- what methods of exchange exist between server and client? in opc ua its change of state - is it right?