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.
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.