What is MP4?
MP4 is the container video actually travels in - what phones record, what messaging apps accept, what every player since the iPod era opens. It usually carries H.264 video, and its one structural quirk matters to converters: the index of every frame lives in a table that has to be exactly right, or nothing plays.
What is MP3?
MP3 is the format that made digital music portable, and thirty years on it is still the safest bet that a file will simply play - on any device, in any car, in any decade-old piece of hardware. It is lossy, and its patents expired in 2017.