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 OPUS?
Opus is the best-sounding lossy codec at low bitrates by a comfortable margin, which is why it carries the audio in nearly every voice and video call you make. It is royalty-free, it handles speech and music equally well, and it is usually wrapped in an Ogg container.