Do flash and fill light mean the same thing?

no! The flash is just a flash, which is bright enough to illuminate. But the fill light is always on, but the brightness is not enough to illuminate, so it can only supplement the light, so it is called the fill light! Of course, it's good to turn off the lights by flashing. Otherwise, why do all professional cameras use flashing lights? Only those mobile phones that have no requirements for photography use fill lights.