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.
GIF is the internet's court jester: 1987 technology that still runs every group chat. An animated image format: every frame is stored with a 256-color palette, there's no audio track and no inter-frame compression. That makes it the right destination when what you need is short, silent, autoplaying loops that work absolutely everywhere.