Multicast: one stream, replicated only where it is requested
Every mechanism has a step-by-step animation, the configuration explained line by line and the exam notes. Where a parameter changes the outcome, you get a simulator where you change it yourself.
TL;DR
The source sends a single stream to a group address (224.0.0.0/4) without knowing who is listening; receivers ask their router for the group with IGMP (or MLD), and the routers use PIM to build among themselves a tree on which the stream is copied only where there are requests. Each hop accepts the stream only on the RPF interface — the one through which it would send unicast toward the source — and that is how loops are avoided. PIM Sparse Mode connects sources and receivers that don't know each other through a rendezvous point (RP), then moves to the shortest path; SSM drops the RP when the receiver knows the source; Bidir keeps a single tree in both directions. On top of these come RP discovery (static, BSR, Auto-RP, Anycast), security (accept-register, boundary, IGMP limits), MSDP between domains, mVPN over MPLS and the multicast underlay of VXLAN. Most incidents are a wrong RPF, a different RP across routers and PIM missing on a hop.
How to read the notes: red = critical for the exam, green = theory, blue = numbers, yellow = pitfalls, cyan = differences between platforms (IOS-XE, IOS-XR, NX-OS), between the RFC and Cisco's implementation, and between IPv4 and IPv6. In the animations, the configuration lines used at the current step are highlighted automatically; the ← → arrows change the step when the animation is focused.
The analogy, in short
Think of a newspaper subscription service: the publisher prints a single edition and doesn't know who reads it; you subscribe with your building manager; the mail carriers make copies only on the streets that have subscribers and accept the newspaper only if it comes from the direction in which they would send a letter to the publisher. Whoever doesn't know where the publisher is asks the city's central office.
The model holds up to §12. At mVPN and NG-mVPN (§13) and at VXLAN (§14) it partially breaks down; see the full analogy and its limits in §18.