Deployment of cell sites at events like stadiums for sporting events and outdoor music festivals is pretty common for years and years. Unless you're a hermit you almost have certainly seen a COW at one point or another. Cf Wikipedia mobile cell sites, especially COW.
I've seen them around alright, just not realized how common or easy-to-set-up they might be for an event like this, or how they work from an inter-provider standpoint. Do they not cost a whole ton for the organization setting up the event, and wouldn't every provider (T-Mobile, AT&T, etc.) be required to set one up? It seems like such a massive headache and cost for something like a Pokemon-catching crowd that might not even generate much revenue(?) that I'm confused how everyone is saying "just deploy extra capacity". Like do providers just follow crowded events for free and put antennas wherever people are as if it's no big deal?