Here's the FFmpeg documentation for x264, an open source h.264 encoder: https://trac.ffmpeg.org/wiki/Encode/H.264
So the encoding strategy is tunable for an encoder, but also there are different implementations that might perform differently. They all produce a h.264 video stream that's decodable by any standard player.
Good to see more coverage on how frail phone numbers are as a security layer, we need to push for more secure ways of doing 2FA.