What is WebM?
Google's royalty-free video container, built for the web and carrying VP8, VP9 or AV1 video with Opus audio. Every modern browser plays it without a plugin or a licence, which is why it's the sensible choice for video you host yourself and a poor one for video you send to someone's phone.
What is Opus?
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. Royalty-free, equally good on speech and music, and usually wrapped in an Ogg container. It always runs at 48kHz internally.