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.
FLAC is the audiophile's handshake: CD-perfect sound at roughly half the shelf space. Free Lossless Audio Codec: think ZIP for sound: it decompresses back to the exact original. That makes it the right destination when what you need is archiving music with zero quality loss and a genuine size saving.