would someone give an ELI5 on how a sand battery works? Is it just purely thermal mass, just with tons of sand?

There's pipework for circulating air inside it when they want to charge/discharge it, but yes, essentially it's mostly tons of sand.

They have resistors for charging it with electricity (resistors heat the air, air is circulated in the pipes which heats the sand) when the electricity price is cheap, and then for discharging they have a air-water heat exchanger so they can pump the heat energy into the district heating network.

You use electricity (ideally cheap solar/wind) to heat air. That hot air circulates through a silo full of sand. The sand holds the heat for months. Later the heat is drawn out and used for buildings or industrial processes.