Hi @Ibrahim Jamil
Actually, the multicast group (called GIPo) is associated to the Bridge Domain, not the EPG - this is because the BD is the L2 domain boundry in ACI.
When BUM traffic is received by a Leaf, the traffic is encapsulated into a VXLAN multicast frame and it is always transmitted to all the the other leaf nodes, using the ftag tree associated to the multicast group. Once received by the leafs, it is then forwarded to the connected devices that are part of that Bridge Domain or dropped depending on the type of BUM frame and on the specific Bridge Domain configuration. If you have a Multi-Pod setup, the same BD GIPo is being extended through the IPN and used to forward the BUM traffic.
More details about the multicast trees in aci fabric: https://www.cisco.com/c/en/us/td/docs/switches/datacenter/aci/apic/sw/1-x/aci-fundamentals/b_ACI-Fundamentals/b_ACI-Fundamentals_chapter_010010.html#concept_1E8BF286E1C043EF839D80935FDF9F50
Stay safe,
Sergiu
