Originally a literal magnesium-powder flashbulb -
镁 (magnesium) burns with a brilliant white light, the basis of pre-electronic photography flashes. Modern cameras use Xenon flashes, but the term has survived as a vivid metaphor for the harsh attention of paparazzi and media.
活在镁光灯下 ('living under the flashbulbs') = 'living in the spotlight'.
