Where WebM files come from
If you recorded something in a browser — a meeting, a voice note, a screen capture from a web tool — the result is very often WebM with Opus audio. Opus is an excellent codec and almost no consumer software outside a browser will open it, which is why this conversion exists.
Because Opus is not AAC, the lossless copy option does not apply here in the usual way: to get a file that a music player or an editor will accept, the audio has to be decoded and re-encoded. MP3 at 192 kbps is the safe choice; WAV if it is going straight into an editor.