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 AAC?

MP3's designated successor and the audio in most video - YouTube, streaming services, broadcast, virtually every MP4. At a given bitrate it beats MP3 audibly, especially below 128kbps, and every phone and browser decodes it.