How does BYD make money on this
The same way Tesla makes money with their own supercharger network - they sell cars and electric via the chargers.
The reason BYD is building these specific chargers is because their future cars will be able to use them to charge at insane speeds but only if a charger can handle it. Right now most "fast" chargers max out at about 350kW, even the bran d new ones. There's a few omising 400kW or even 500kW but BYD is marketing cars that can do 1000kW.
They need some chargers to exist so they can sell those cars, it's better for them to start building it than waiting years for the rest of the industry to catch up.