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.
Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a WEBM file above and choose afterwards.