The fire hydrants that are in cities and towns are connected to the same municipal water supply that the homes and businesses are connected too.
When a firetruck gets hooked up to a fire hydrant the fire hydrant should never run out of water unless the water tower or other supply of water is cut off or runs out.
Water towers store a lot of water to supply the cities and towns and there's usually more than enough water supply to the fire hydrant to allow the fire department to fight the fire without running out of water.