As far as we know it was a mod from modland, and nothing to do with BeamMP (luckily). We don't know which mod, but I've seen people on twitter talk about reverse engineering it - maybe someone knows which mod?

And yes when people join modded servers (which is clearly marked and the mods are listed), the mods are downloaded and installed. This enables super cool community made gamemodes, some of which have thousands of players.