What is MOV?
MOV is QuickTime's container - what iPhones and Macs record. The open secret is that MP4 was standardised *from* it, so the two share their internal structure almost exactly, and reading one is reading the other. Writing a true MOV is another matter, which is why it converts from here but not to.
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.