Hi Azm,
Multicast isn’t really used on the Internet. It would make a lot of sense to use multicast for online radio but in reality, it’s unicasted everywhere. There is no “global” multicast network that spans multiple ISPs. One application that was common back in the days on the LAN was Norton Ghost to send a system image to all computers. Without multicast, you had to unicast huge images to all computers which wasn’t very efficient.
When your computer wants to receive a multicast stream, it uses IGMP to “report” which multicast group it wants to receive. IGMP version 2 is very common but it doesn’t allow you to specify a source which means that everyone can send packets to 239.1.1.1. With IGMP version 3, you can specify the source which means you can report that you only want to receive traffic for multicast group 239.1.1.1 from source x.x.x.x.
Once a multicast enabled router receives your IGMP packet, it’s up to PIM. It really depends if you are using PIM dense or PIM sparse mode how the multicast traffic is flooded in the network. PIM dense uses a “flood and prune” model while PIM sparse uses a “join” mechanism to request the multicast traffic from an upstream router. These two lessons explain that process:
https://networklessons.com/multicast/multicast-pim-dense-mode/
https://networklessons.com/multicast/multicast-pim-sparse-mode/