How does the receiver in a multicast scenario know which IP address it should listen to?

A server is streaming videos and the source IP of the packet is 192.168.1.1 and the destination multicast IP is 239.1.1.1.

Question 1. Who decides which multicast IP to use? Is it decided by Server or clients who listen to it?

• If it is by the server -> Then how do clients come to know about which Multicast IP it should listen to?
• If it is by the clients -> How does this work?

The server/sender sends to a multicast group (multicast ip and udp port). Multicast is one-way, udp, traffic. As for determining the group (ip:port) to use, it would need to be known or learned (possibly by dns resource records) or a configuration file. Also there are some specific ranges that are link-local and/or are reserved or limited in scope and reach.