WebM does not play on many phones, TVs or in most video editors. AAC is more efficient than MP3 at the same bitrate. WebM is at home in royalty-free video on websites; AAC is the usual choice for efficient audio for streaming and video soundtracks. The AAC contains the soundtrack only; the picture is dropped.
The defaults suit most files. Under Options you can choose the bitrate and trim the start and end. This is demanding work for a browser: a long or high-resolution file can take several minutes, and your device does all of it, so keep the tab open.
The conversion uses your browser's own video and audio codecs (WebCodecs), with hardware acceleration where the device has it; the file is read from disk as it goes and can be up to 2 GB. Where the browser lacks a codec, the page falls back to the compatibility engine. AAC is written at 192 kbps unless you choose another bitrate.