FLV is a relic of the Flash era: the format YouTube grew up on, stranded when Flash died. It was born in 2002, Macromedia (later Adobe), for Flash Player, and its real weak spot is playing anywhere at all since Flash was switched off in 2020.
MP3 is the format that taught music to travel: small, scrappy, and welcome absolutely everywhere. MPEG-1 Audio Layer III. Perceptual coding: it throws away the parts of the signal your ear is least likely to miss. That makes it the right destination when what you need is playing on anything with a speaker, from new phones to twenty-year-old car stereos.