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 AIFF?
AIFF is Apple's uncompressed audio format - WAV's counterpart from the Mac side of the world, and still what Logic and older Pro Tools sessions use. It stores its sample rate as an 80-bit extended float, a piece of 1980s exotica most converters get wrong.