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 WEBM?
WebM is the web's own video container - Matroska slimmed down to the royalty-free codecs (VP8, VP9, AV1). Browsers play it natively and screen recorders love emitting it; the rest of the world's video tooling is distinctly lukewarm about it, which is why it so often needs converting.